Prune MeldParser to essentials (hide on Python >= 2.7).

- Move check for duplicate meldids into the TreeBuilder, where the
  API wants it (Python 3.3 doesn't call our '_start_list').

- Drop custom '__init__', '_start*', and 'close' accordingly.

- Just use stock XMLParser for Python 2.7+, because it already does
  what we need for comments.

Tests now pass on Python 3.3.