Add support for Excel output using xlwt.

This request adds experimental support for the xlwt library.  Related to issue#6.  The changes should be backwards compatible with pyExcelerator, but xlwt will be chosen by default if available.

There's a couple of minor improvements over the pyExcelerator code.
  * Heading text is output in bold face.
  * Cell widths are autosized in a rudimentary fashion to match the cell data.

The xlwt output doesn't support multiple output files as appears the original intent for the pyExcelerator implementation.