Writing metadata

[jvoisin]

It would be nice to be able to write metadata to files, and not only reading them.

[Diaoul]

With #5 implemented we can add setters for the properties so the element is edited easily with the high level API.
As for writing back the elements to the file, this will require some work with the ebml parser: https://github.com/Diaoul/enzyme/blob/master/enzyme/parsers/ebml/core.py
I think a dump method in Element that is recursivly called is the way to go. Also need to implement WRITERS as we already have READERS.