Adds ArrayAccess support to workbooks & worksheets

Usefull for when needed access to Excel as an array. It references
worksheets inside a workbook by name, the same apply to cells, always
use coordinates ('A1', etc..) or sheet names when using the array access
capabilities.

Changes:
    * Implements ArrayAccess interface
    * Add unit test for it.
Merge branch 'develop' of https://github.com/davidmpaz/PHPExcel into develop

Conflicts:
	Classes/PHPExcel/Worksheet.php