Comparison operators on strings are usually case insensitive

Excel, Gnumeric and Google Spreadsheet are case insensitive, so the
default behavior of PHPExcel is modified accordingly.

However OpenOffice is case sensitive and is also supported via the
compatibility mode of PHPExcel.

Fixes #31
Merge pull request #270 from PowerKiKi/issue_31

Binary comparaison of strings are case insensitive