Add support for @required attribute on a parameter

Jersey (or JAX-RS) does not currently allow us to set whether a parameter is
required or not. It is difficult to make assumptions on whether a param is
required (even in the case of form params since regexps can be used in clever
ways). It would be useful if parameters could be annotated with @Required and if
the parameter is absent a 400 response will be returned.

This should also be set in WADL generation.
#### Environment
Operating System: All
Platform: Macintosh