Avoid missing Queue.Empty reference at process termination

Fixes #88
Stop and join the workflow thread to avoid exceptions when daemonized threads are shutdown at process termination
local pdiff
demo of local pdiff
tweaks
Merge pull request #90 from danvk/localpr

A tool to run local perceptual diffs
pip-installable local pdiff tool.

This includes a few new command line flags (test_filter, list_tests) and
a demo of how the tool works.
merge upstream
Merge pull request #91 from danvk/pip2

dpxdt pip package for local pdiff testing
support customer user agent in config.
Merge pull request #92 from siderakis/userAgent

support custom user agent in config.