[Fixes #739] Attribute Routing: Multiple routes per-action

1. Support multiple [Http*] attributes on an action.
2. Support multiple [Route] attributes on a controller and on an action.
3. Support creating multiple attribute routes using [AcceptVerbs("...", Route = "...")]
4. Detect attribute routed actions during action discovery and return one action per [Http*],
   [Route] or [AcceptVerbs] attribute found on the method when there is at least one valid attribute route.
5. Merge all the HTTP methods of [Http*] and [AcceptVerbs] attributes in a method during
   action discovery when there are no valid attribute routes defined on the action.
6. Build one action descriptor per controller [Route] + action [Http*], [AcceptVerbs]
   or [Route] combination in an action.
7. Disallow the use of attributes that do not implement IActionHttpMethodProvider and
   IRouteTemplateProvider simultaneously in methods that define attribute routed
   actions and throw an exception during startup.
8. Disallow mixing attribute routed and non attribute routed actions on the same method
   and throw an exception during startup.