Add _get function to base.Manager

This is useful so that we don't have to use _list in each Manager
subclass for getting a single resource and then fetching first item of
the array and checking for index error. This method wraps it for us.
Rack class

Currently just inherits from Resource and doesn't contain anything
testable really.
RackManager class
Merge pull request #1 from jistr/ftr/racks_python_api

Racks Python API and _get in base.Manager