Constructor can accept request class name

The optional request_class constructor allows a class that inherits from
Plack::Request to be specified in the Web::Machine constructor.
Doc request & reponse in Web::Machine::Resource
Bump version
Add Raynham
v0.15

    MISC:
        - The Web::Machine class now accepts an addition parameter,
          request_class. This allows you to use a custom subclass of
          Plack::Request. (Mike Raynham)
        - Documented the request and response methods in
          Web::Machine::Resource. Requested by Olaf Alders.