add the ability to add untyped extended data / name value pairs

Hi Christian,

As discussed a while back I started the work to get extended data into collective.geo.kml using adapters. First up naturally is fastkml.

This pull request solves the simplest extended data case with untyped name / value pairs (see https://developers.google.com/kml/documentation/extendeddata).

I'm not too familiar with KML or lxml/etree, so I would appreciate if you could have a look at my implementation. It seems to work and passes my tests, but I'm not exactly sure that I got it right.

I put the extended data stuff into kml.py for now. I think it makes more sense to move those to a separate file once the other kinds of extended data are supported. But do let me know if there's anything you want done differently.

Cheers,

Denis