Implement route_prefix for Flask adapter

[soundofjw]

Currently, if an endpoint has a route URI of /space/endpoint, there is no way of setting a common prefix such as /api/space/endpoint.

This is exemplified on GAE by having contradicting .yaml route that differs from the endpoint URI.

[rknLA]

This is handled by the route_prefix when you bind pale to an adapter.

The example app test case shows an example of this, and you can also have a look at the lines that make this happen for webapp2 and for flask...

Well... the flask one isn't implemented yet.. :X