Implemented Read-Only file opening mode
Merge branch 'master' of https://github.com/pysco68/LinqToExcel into pysco68-master

Conflicts:
	src/LinqToExcel/ExcelQueryFactory.cs
	src/LinqToExcel/Query/ExcelQueryArgs.cs
	src/LinqToExcel/Query/ExcelUtilities.cs
added tests for ReadOnly mode and refactored the code