The intended behavior for Sink.get_source_cap() when no
Source_Capabilities are read was to return an empty list.  This wasn't
done before, instead raising a ValueError when read_pdo() tried to parse
"No Source_Capabilities" as an UnknownPDO.  Now we're a bit more
cautious, and the bug is fixed.

Whoops!  The version that was pushed to PyPI is correct, but I forgot to
commit the change.

The library has now been tested with Python 2.7, 3.3, 3.4, 3.5, and 3.6.
The README and setup.py now indicate this.  The README also says how to
run the tests, and that you need a PD Buddy Sink plugged in for them to
be completely thorough.

May change before the real 0.1 release, but this allows easier test
coverage measurements.