compatibility with SQLAlchemy

Aims of this pull request is to start working on:
 * clarify if HSTORE_GLOBAL_REGISTER should be used in deferred registration too (I think so)
 * determine if HSTORE_GLOBAL_REGISTER is the right settings name for the performed action
 * determine a good name for the setting that can turn off globally=True
 * SQLAlchemy compatibility #45 
It can be changed to more specific setting name like `DJANGO_HSTORE_PSYCOPG2_REGISTER_GLOBALLY` ?