Performance Issue CellToText

I discovered a serious issue with the CellToText method when it comes to performance. By caching the shared strings, performance on a large dataset went up from 55000ms to 500ms.

Performance comparison:

![image](https://cloud.githubusercontent.com/assets/956362/4148649/bdead24c-3428-11e4-8f84-910a536e35b3.png)