HasOnePolymorphic and HasManyPolymorphic with tests

Hi,

I'm converting a rails application to PHP and it has a few polymorphic relations so I added the functionality to the ORM.

I have only added HasOnePolymorphic and HasManyPolymorphic relations but I'll be happy to add BelongsToPolymorphic and ManyToManyPolymorphic relations if needed.

Everything is tested and 100% compatible with existing code.