Make couchmodel work with Rails 3.2

I am using the couchmodel gem in a Rails project. With Rails 3.2, couchmodel no longer works, because

* active model accesses the attributes method eagerly
* active model defines methods that lead couchmodel to the false conclusion that the key_accessor methods conflict with a user method

I have a fix (at least, here it works), I'd like you to pull.

* It replaces the attributes attr_reader with a method with {} default
* It removes the method_defined check in key_accessor, as the key_reader/key_writer overwrite the existing method, which is ok, I think