Support UserDict and friends

[acetylen]

Currently (version 0.3.0) the qtoml dumper doesn't understand UserDicts, which makes using their subclasses annoying. Subclassing dict would of course work, but the internal optimizations of the dict class means that overriding methods can be unpredictable.

[miccoli]

Also the closest relative to qtoml in the standard library, namely json, does not support UserDict objects:

>>> json.dumps(UserDict(foo='bar'))
...
TypeError: Object of type UserDict is not JSON serializable

May be the best solution would be to implement an extensible TOML encoder, see #11.