Add support for Excel output using xlwt.

The xlwt library is available from https://github.com/python-excel/xlwt.
Fall back to using pyExcelerator if xlwt is not available.
Don't run Excel test cases if no Excel support.
Update README to describe xlwt support.
Merge pull request #7 from austinphillips2/xlwt-support

Add support for Excel output using xlwt.