Features/protobuf header

This brings heka-py up to date with hekad 0.20

Changes in a nutshell:

* wire protocol now uses the protocol buffer header format
* UDP and TCP are both supported
* JSONEncoder and ProtocolBufferEncoder are supported for the payload
* documentation has been updated to remove the old ZeroMQ senders, Mozilla Sandstone theme is now used and all configuration has been retested with the new API
* HMAC signatures are now supported