[Feature] morphTo eagerload support

Hey guys,

I needed eagerload support for a morphTo relationship, I stumbled upon a lot of other people needing it (issue #1681, http://forums.laravel.io/viewtopic.php?pid=56917), and I thought I'd give it a go to implement it.

Please note that this is an initial proof of concept / draft, but it actually seems to work!

I am hoping somebody can take this code and straighten it out, or tell me how I can make it better.