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.
Add waitFor stanza to local dpxdt tool YAML
Add screenshot retry logic
Bump PyPI version
Merge pull request #96 from danvk/waitfor

Add a "waitFor" stanza to local dpxdt command line tool
Return meaningful exit code from "dpxdt test" command.
Merge pull request #100 from danvk/exit-code

Return meaningful exit code from "dpxdt test" command.
bump version
Add a flag to upload screenshots to imgur.

This is helpful when running on Travis-CI, where you can't access files
on the local worker.
Merge pull request #101 from danvk/master

Add a flag to upload screenshots to imgur.
Add a simple way to wait for an URL to resolve
Make setup script its own process group
Carve up RunTestsWorkflowItem into smaller WorkflowItems
Merge pull request #105 from danvk/master

API, robustness and cleanliness improvements for local dpxdt
version bump
Merge branch 'master' of https://github.com/bslatkin/dpxdt