Implements ArrayAccess for workbooks and worksheets

Hello,

First of all thanks for this project, it is great. Next, I wanted to add ArrayAccess support because recently I came up with a situation of needing it when accessing the workbook/worksheet data. For example when using [Ruler](https://github.com/bobthecow/Ruler), it expects an array as context for evaluating its rules. Instead of building the array from the excel object, I wanted to pass along the PHPExcel instance itself and let it provide necessary interfacing for getting/setting its data. I would like, if it is inline with the current development, to have this feature in the main stream.

Thanks for listen,
David