Allow to define what methods an Crud action can be requested as

Refs #49 

Things I'm undecided about:

* should there be a config for API requests and one for non-API requests?
* requestMethods doesn't merge settings, but always override, is this ok ?
* are the defaults in the actions sane?
* right now the API listener does the enforcement, this could be moved to CrudAction too if a normal non-API request should be able to enforce the request verbs to allow