Add support for adding multi-column conditions into queries
Added a protected method for filtering multiple column primary keys
Multiple column names can now be specified when calling get()
where_*() methods are now compound primary key aware

Also, now they allow multiple column-names per call.
having_*() methods are now compound primary key aware

Also, now they allow multiple column-names per call.
UPDATE queries are now compound primary keys aware
delete() method is now compound primary keys aware
INSERT queries are now compound primary keys aware
Tests for compound primary keys queries added
Document the new compound primary keys behaviour and features