Add support foreign_key to `one` associations

You can set a foreign_key to an stored_key association with the option :foreign_key => :key
Add inverse relation, for relations without persist a foreign key.

The relation needs an index on the foreign key
Support `many` association with foreign key option
Add inverse relation, for `many` associations.
Merge pull request #1 from citrusbyte/support_associations_with_foreing_key

Support associations with foreign key