Add hydra rights to bdrxml/rights.py
Added hydra rights schema and builder.
Get the builder back from a rights instance. Also get the indexer from
the other format.
Cleaned up pull tests based on pull request feedback.
Added some docstrings on main files.
Merge pull request #19 from jrhoads/f-hydra-rights

Add indexing, hydra rights, and a converter between bdr rights and hydra rights.