Pyexcelerate

This uses PyExcelerate to generate an XLSX file. I have checked in a modified copy of the library as a temp measure.

Still need to do many of the things in the other pull request as the code still doesn't handle errors during the creation of the output files correctly.

Don't think we should deploy until Monday. If people want to test it out it has been deployed on in the "Pycelerate Test" tool in the ScraperWiki Test datahub @pwaller @paulfurley @drj11.