Implemented drop_foreign_key.
Make foreign_key_name raise unless unique matching key is found.
Make add_foreign_key() reversible.
Added spec tests.
Merge raxoft's drop_foreign_key commit
Merge the drop_foreign_key alter_table generator method

Add and modify specs to cover the foreign_key_name method.

Update related documentation.

Make SQLite handle drop_foreign_key by only dropping the
related foreign key constraint instead of all foreign keys
contraints.