Auto id generation when no id is supplied

This is my first stab at id auto-generation. 
Ids are created using the [cuid](https://github.com/dilvie/cuid) library. 
I also modified `post()` and `postSync()` to return the id of the object as well.

I wrote a couple tests to cover these cases. If you want more coverage, let me know.

If there is anything you would like changed, please let me know.

--- rooftopsparrow