Add support for stomp-1.1

I have tested this locally in the test suite as well as with an activemq broker
and my own application.

I think I handled backwards compatibility well.  Users of stomper that want to
stay on stomp-1.0 should not notice any difference.  To use stomp-1.1, you have
to ``import stomper.stomp_11 as stomper``.  If we want to switch it out as the
default at some point, that could be done in ``stomper/__init__.py``.

(Bookkeeping - this relates to #6)

Here's a link to the protocol specification:  https://stomp.github.io/stomp-specification-1.1.html