Support for PDF format

We've been exploring different options for parsing PDFs. Currently we're using an (alpha) in-house library called [pdftables](http://github.com/scraperwiki/pdftables/) (we blogged about it [here](http://blog.scraperwiki.com/2013/07/29/pdftables-a-python-library-for-getting-tables-out-of-pdf-files/))

This pull request integrates pdftables into messytables. It is an optional requirement - if pdftables is not installed, messytables will work as usual and the PDF tests will be skipped.

We're looking into other ways of extracting tables from PDFs, but either way we'll need the messytables integration.