Support for dBase (DBF) files

The [dBase](http://en.wikipedia.org/wiki/Dbase) file format is pretty antiquated, but it still has quite a hold in the GIS world where it has become the tabular format used by ESRI's GIS vector format in ArcGIS.  Due to the prevalence of the DBF tabular format in GIS circles, interoperability with other, more common tabular data formats would be incredibly helpful for us in the GIS world.

This pull request utilizes version 2.2.5 of [dbfpy](http://dbfpy.sourceforge.net/) to provide python2 support for DBF files, and my own adaptation of dbfpy for python3 (which I now also have committed to my own [github account](https://github.com/phargogh/dbfpy3) in addition to providing it here in tablib).

All tests, including those I have added to test_tablib.py, pass in both python2 and python3, but I have not tried to run the tests with versions of python below 2.7.

Let me know what you think, and thanks for your work maintaining this package!