Added override for creating custom eloquent/model collection

[bbrothers]

Eloquent allows us to override the default Collection class for a given model by implementing a newCollection method in the model.

This merge request makes a minor change to allow that same functionality but default to an instance of Jenssegers\Mongodb\Collection.

[coveralls]

Coverage increased (+0.0%) when pulling 2475dd6 on bbrothers:custom-eloquent-collections into 3bddd4b on jenssegers:master.

[philipbrown]

Hey @jenssegers any reason why you haven't merged this? ☺️

[ryangoodrich]

+1 to have this merged in to the next release