Now using a block to return the params dictionary

Hmmm... I'm not quite sure if this is all that buildery patterny ... really it's just building the params hash in a block and passing it through.. so that could equally be just built outside the call to addCarpool and passed in as a params hash.

On the plus side, it is a lot less lines of code, and it's easier to get at at the params as its in the context of the AddCarpoolController.

Anyway, can discuss! :)