Renaming request_parser to request_factory.
Renaming request_parser to request_factory.

Details:
* This is more descriptive of what is going on. We're actually
  building a request from an input HASH, not really parsing data
  in the strictest sense.
* Removed the parse_request class method. Instead, moved the
  building into the 'build' class method, and put the validation
  into its own instance method.
* SIDE EFFECT: Implementers can override the validate_request!
  method to do additional (or no) validation of the request itself.
  Implementers SHOULD still validate the wholeness of the Command
  in (which may cover more than the request, e.g. db connection settings)
  in the initializer, or another 'validate!' method.

byu/serf