Add 'except' and 'only' methods to Eloquent Collection class.

This pull requests adds `except` and `only` methods to the Eloquent `Collection` class. 

**Use case**: I am currently working on a system that has projects. Each of these projects have multiple clients, one of which is the 'primary client / project owner'. It's more efficient to pull in all these clients in one query and then filter the collection when needed, than to fire two queries. With these methods I can easily filter the collection by either excluding the primary client from the collection, or only fetching that client from the collection.