Allow for manual calculation of the dimensions of a sheet if no DimensionRef is

provided in the worksheet.  Issue 30 occurs because this element is missing
from Google Docs XLSX output.

lib.go

@@ -227,6 +227,41 @@ func getMaxMinFromDimensionRef(ref string) (minx, miny, maxx, maxy int, err erro
 	return
 }
 
+// calculateMaxMinFromWorkSheet works out the dimensions of a spreadsheet
+// that doesn't have a DimensionRef set.  The only case currently
+// known where this is true is with XLSX exported from Google Docs.
+func calculateMaxMinFromWorksheet(worksheet *xlsxWorksheet) (minx, miny, maxx, maxy int, err error) {
+	// Note, this method could be very slow for large spreadsheets.
+	var x, y int
+	minx = 0
+	miny = 0
+	maxy = 0
+	maxx = 0
+	for _, row := range worksheet.SheetData.Row {
+		for _, cell := range row.C {
+			x, y, err = getCoordsFromCellIDString(cell.R)
+			if err != nil {
+				return -1, -1, -1, -1, err
+			}
+			if x < minx {
+				minx = x
+			}
+			if x > maxx {
+				maxx = x
+			}
+			if y < miny {
+				miny = y
+			}
+			if y > maxy {
+				maxy = y
+			}
+		}
+	}
+	return
+}
+
+
+
 // makeRowFromSpan will, when given a span expressed as a string,
 // return an empty Row large enough to encompass that span and
 // populate it with empty cells.  All rows start from cell 1 -
@@ -316,7 +351,11 @@ func readRowsFromSheet(Worksheet *xlsxWorksheet, file *File) ([]*Row, int, int)
 		return nil, 0, 0
 	}
 	reftable = file.referenceTable
-	minCol, minRow, maxCol, maxRow, err = getMaxMinFromDimensionRef(Worksheet.Dimension.Ref)
+	if len(Worksheet.Dimension.Ref) > 0 {
+		minCol, minRow, maxCol, maxRow, err = getMaxMinFromDimensionRef(Worksheet.Dimension.Ref)
+	} else {
+		minCol, minRow, maxCol, maxRow, err = calculateMaxMinFromWorksheet(Worksheet)
+	}
 	if err != nil {
 		panic(err.Error())
 	}

lib_test.go

@@ -306,6 +306,52 @@ func (l *LibSuite) TestGetMaxMinFromDimensionRef(c *C) {
 	c.Assert(maxy, Equals, 1)
 }
 
+func (l *LibSuite) TestCalculateMaxMinFromWorksheet(c *C) {
+	var sheetxml = bytes.NewBufferString(`
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<worksheet xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main"
+           xmlns:r="http://schemas.openxmlformats.org/officeDocument/2006/relationships"
+           xmlns:mx="http://schemas.microsoft.com/office/mac/excel/2008/main"
+           xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+           xmlns:mv="urn:schemas-microsoft-com:mac:vml"
+           xmlns:x14="http://schemas.microsoft.com/office/spreadsheetml/2009/9/main"
+           xmlns:x14ac="http://schemas.microsoft.com/office/spreadsheetml/2009/9/ac"
+           xmlns:xm="http://schemas.microsoft.com/office/excel/2006/main">
+  <sheetViews>
+    <sheetView workbookViewId="0"/>
+  </sheetViews>
+  <sheetFormatPr customHeight="1" defaultColWidth="14.43" defaultRowHeight="15.75"/>
+  <sheetData>
+    <row r="1">
+      <c t="s" s="1" r="A1">
+        <v>0</v>
+      </c>
+      <c t="s" s="1" r="B1">
+        <v>1</v>
+      </c>
+    </row>
+    <row r="2">
+      <c t="s" s="1" r="A2">
+        <v>2</v>
+      </c>
+      <c t="s" s="1" r="B2">
+        <v>3</v>
+      </c>
+    </row>
+  </sheetData>
+  <drawing r:id="rId1"/>
+</worksheet>`)
+	worksheet := new(xlsxWorksheet)
+	err := xml.NewDecoder(sheetxml).Decode(worksheet)
+	c.Assert(err, IsNil)
+	minx, miny, maxx, maxy, err := calculateMaxMinFromWorksheet(worksheet)
+	c.Assert(err, IsNil)
+	c.Assert(minx, Equals, 0)
+	c.Assert(miny, Equals, 0)
+	c.Assert(maxx, Equals, 1)
+	c.Assert(maxy, Equals, 1)
+}
+
 func (l *LibSuite) TestGetRangeFromString(c *C) {
 	var rangeString string
 	var lower, upper int