Features/protobuf header

Alright, this should be a working version of heka-node for 0.2.

Changes include:

- hekad 0.2.0 support
- msgpack has been dropped
- ProtocolBuffers is now a required dependency.
- The new wire format is basically a protobuf header, and either a
  JSON or Protobuf encoded payload.  
- JSON encoding of the payload has been changed for hekad 0.2.0

Things to note here:

The JSON encoding of payload is the only one that works.  I'm getting bizarre encoding problems with protobufjs and serializing doubles like the sample_rate in incr().

Testing of the HMAC has been done by manually inspecting the byte array in the header.