Add lists function for Eloquent query builder. Fixes #668

Implemented the `lists()` function on the Eloquent query builder. This fixes the problem where Model accessor mutators weren't called.

I must admit, I was writing a couple of unit tests but I'm not yet very experienced with them so they kept on failing. Can anyone perhaps help me out with them? There should be a unit test in `tests/Database/DatabaseEloquentBuilderTest.php` for testing basic lists function and one in `tests/Database/DatabaseEloquentModelTest.php` for testing mutators. Any help is greatly appreciated! :)