Alternate method definitions

After today's DWG call, I took a closer look at the methods definitions. In short, something did not sit right with me for defining a pair or related records and relying on documentation and convention to relate the two concepts. 

I took a stab at re-writing the `methods.avdl` using [Avro's IDL syntax for writing RPC messages](http://avro.apache.org/docs/current/idl.html#define_messages).  Please note that the messages can be [handled by different wire formats](http://avro.apache.org/docs/current/spec.html#Protocol+Wire+Format) including HTTP request/response. 

Overall I think that this approach is a better fit for what we need of the format and specification. Thoughts?