Make buildable on Windows + goapp

Go on Google AppEngine restricts importing syscall. It causes build failure testing applications using go-colorable (especially when using a framework using go-colorable) on Windows.

colorable_others.go

@@ -1,4 +1,4 @@
-// +build !windows
+// +build !windows OR appengine
 
 package colorable
 

colorable_windows.go

@@ -1,3 +1,5 @@
+// +build !appengine
+
 package colorable
 
 import (