HTMLTableSet

Hi, here's a HTML Table Set importer for messytables.

It's not fantastic yet; but it's a pretty good start

* Supports rowspan/colspan - currently by inserting blank cells.
* Supports multiple TABLE elements - but may have unexpected behaviour where there are nested tables.
* Doesn't attempt to handle tables that aren't using TABLE, TR, TD, TH.
* Not enormously well tested, but seems to work on the tables I've fed it so far.
* Requires lxml.

It's the first time I've ever made a pull request; let us know if there's anything we can do to improve it for you.