ENH: Support for PyExcelerate as an Excel writer engine.

This is an initial patch to add PyExcelerate as an Excel writer engine.

**It isn't suitable for merge** yet since there is issue with date handling in PyExcelerate that I am working to resolve with the authors. I just want this to appear on the pandas timeline for now so I can refer to it in other PRs.

1. The requirements files are missing since the PR will require a PyExcelerate update and release. Also, the `print_versions.py` entry.
2. There is also a failing test due to versioning.
3. There is currently no merged cell support.

I'll fix all these before the final PR request. 

John