Add docs target to Makefile. Reorder targets to match python-hl7.
Abstract out message parsing and ACK generation.

Move all receiver related code into receiver module.
Add a concrete implementation that uses new python-hl7 ACK creation.
Now requires updated python-hl7 0.3.1 with ACK functionality.
Update docs, add API docs.

autodoc IHL7Receiver zope interface using repoze.sphinx.autointerface sphinx extension.
Use factory defaults for getCodec().
Makefile docs target a function of env (get requirements.txt updates)
Ignore docs/_build from git
Make the message return type a class-configurable variable by default.
Add make clean target
Rename *ParsedMessage->*MessageContainer, unparsed_message->raw_message, parsed_message->message_container
Improve "make clean", "xargs -r" is a GNU extension not in MacOS xargs.
Adjust docs.
Merge branch 'hl7-ack'