Initial implementation for #29
Capture and re-raise error when base request is not found
Raise RequestAlreadyDefinedError
Refactor define_request and remove recursive method call
Refactor variable names
Avoid module duplication voodoo and just use unique names per example
Load base request class as needed
Convert list_hosts to use request inheritance
Update inheritance DSL to be more explicit

Convert the inheritance DSL so that it expects a hash for options which
includes an inheritanc key-value pair. This makes the inheritance
directive more explicit when someone view's the request file's code.
Rename url_for to base_url_for
Update request files to use inheritance
Update metering and volume request files to use inheritance.
Merge pull request #51 from relaxdiego/fix/29

This implements the request definition inheritance feature 
(solution 2) as defined in issue #29.