Adds create method to ResourceClassManager

This is simplest implementation of create method
for ResourceClassManager
It just takes keyword arguments and pass them
to the _create method of base.Manager as a dictionary
Merge pull request #7 from petrblaho/ftr/resource-classes-create

Adds create method to ResourceClassManager
Add _single_path method for single resources path

It makes sure that resource id is not None, so that e.g. DELETE request
is not accidentally called on a collection URL.

Also _path method stub is now in base.Manager, to be overriden by child
classes.
Rack delete
Use _single_path in managers' get methods