Agent updates

 - Implemented DSL for specifying mib providers
   + The DSL methods are implemented in the RequestContext class.
     Objects of this class are used to instance_eval all get/set/get_next/get_bulk
     procs from the providers when requests are received.
 - Added example of simple get/set capable agent
 - TODO: Using a trivia algorithm to match request varbinds to providers right now...
   May be fine, but should consider something a little faster in case of large
   requests and large numbers of providers.