Let the user know if a method has no attribute

Throw a `NotImplementedException` when someone calls a method that we
won't have an implementation for.
Retry count inside the goto was retrying forever
Allow completely empty URL template

This is needed (in conjunction with #40) for generic interfaces to be of
any use at all.
Support for generic interfaces

Fixes #63.

Now people can have all the boring CRUD Web APIs they want and they only
need one interface. :trollface: