Trial of coherence didl_lite and test

This is experimental - not to be merged permanently, but for demo purposes only.

I have been playing with the didl-lite parser from the coherence and brisa projects, in response to recent discussions, to see if they might be useful at all.  Although the one from brisa is older and not recently maintained, it is quite easy to follow, and the coding is explicit and easily extendable.  The code from coherence is more tightly coupled with the rest of coherence, and has a lot of stuff we wouldn't need.

It looks to me as though it provides just about everything we need in terms of didl parsing. For example, it creates the correct subclass (MusicTrack, MusicAlbum, Genre etc) automatically from the XML.

I have changed it very little (just a few lines to get it to work with SoCo, and removed some video stuff) and created a demo script in the root directory called `didltest.py`.  Have a look, and run `didltest`.

@kennethnielsen : particularly keen to hear your views on whether/how we might use any of this, as our resident didl expert.

Lawrence