Implement IncrementalCallback and IncrementalDecoder

This pull request will track completion of issue #9 
 - [x] add IncrementalCallback type
 - [x] update Contracts to parse IncrementalCallback
 - [x] make MethodHandler extensible (in support of IncrementalCallbackMethodHandler)
 - [x] design and implement IncrementalDecoder
 - [x] design and implement async dispatch model