use HSTORE_GLOBAL_REGISTER also when registration is deferred
introduced DJANGO_HSTORE_ADAPTER_REGISTRATION

backward compatibility with the old setting DJANGO_HSTORE_GLOBAL_REGISTER is mantained.
The setting adds compatibility with SQLAlchemy.
closes #45
updated .coveragerc
minor improvements suggested by prospector/landscape
Merge pull request #50 from djangonauts/features/sqlalchemy

compatibility with SQLAlchemy