Use functional tests, python3 support (tox + py.test + lrzsz package)

This implements travis-ci integration with tox and lrzsz package.

Once I added tox support, I added python3 testing -- which immediately failed.


I spent some time adding python3 support.  Some of the compatibility of using
bytearray() around str-data may incur some small overhead.  We can address
these when we decide on a new API for YMODEM (multi-file, file-like iface)
support.

Additionally, debug logging is less verbose in the case of not logging each
successful checksum -- and more verbose in the case of indicating each and
every step through the transfer, and indicating the cause of various failures.

These were decorated as I went with the python3 porting process.