Add keyBy method to Collection

Adds a new `keyBy` method to `Illuminate\Support\Collection`, so that you can do something like this:

    $roles = Role::all()->keyBy('name');

which would result in an array similar to this:

	[
		'admin' => ...model...,
		'editor' => ...model...,
		'user' => ...model...,
	]

For reference, [see this question on StackOverflow](http://stackoverflow.com/questions/24130507/eloquent-column-list-by-key-with-array-as-values).