Implement creating collection with existing model objects.

Current spec status: 45 examples, 0 failures in 0.04132 seconds.
Refactor generic ActiveModel impl. to separate class.

Current spec status: 45 examples, 0 failures in 0.03652 seconds.
Implement #create class method.

Current spec status: 51 examples, 0 failures in 0.08763 seconds.
Implement ::create!, #save! methods.

Current spec status: 68 examples, 0 failures in 0.10712 seconds.