More formats and better API

New things:
 * Added lmxl: XML/HTML parsing
 * Changed the API to better mimic NIF's
 * Better form validation
 * Using South
*WARNING*: The new model is not compatible with the old one, and I do
not provide the South Migration for it (it involves primary keys, and
it's a mess)

The implementation of the forms is rather clumsy but it works well to
generate the HTML forms and validate API calls. It could use some
refactoring, though.