add AttributeList to model.__all__, sphinx configuration
refactor and simplify code generation

new templates, resolvers dynamic loading, entry_points
Merge branches 'develop' and 'master' of https://github.com/olaurino/volib into develop
improve imports in generated code

Also fix indentation in jinja2 templates.
complete basic code generation

complete standard ivoa model (ivoa does not get
generated, but is built-in)

create Reference DM with nested packages, borrowing
concepts from SourceDM and STC.
create reference votable serialization, python objects hooks

volib.get_object returns a python element given its utype
and the version of the model.
populate README.md

some fixes as issues were found by exercising the API
described in the documentation.
Merge pull request #2 from olaurino/develop

Develop