Updates and testing for Chrome and Firefox

[jmbowman]

This resolves some errors I was getting with recent versions of Firefox and geckodriver, and also addresses #60 .

• Default to using operations from the W3C Web Driver spec to get dimensions and take element screenshots, falling back to the older Selenium commands if necessary (geckodriver has indicated that it only intends to support the actual spec, not the legacy Selenium commands).
• Work around a Selenium bug introduced in April 2015 that sometimes leaves orphaned phantomjs processes behind.
• Updated the tox configuration to support testing Chrome, Firefox, and/or PhantomJS.
• Test both PhantomJS and Firefox in Travis (Chrome should be possible also, but I don't want to attempt that many changes to the Travis configuration at the same time).
• Fixed some tests that would fail if run in isolation, only passing when other tests created some directories for them.
• Stopped testing Python 2.6 in tox, since Travis wasn't testing it and even pip and setuptools no longer support it.

[jphalip]

This is great, thank you. Would you mind just breaking this up in separate logical commits?

[jmbowman]

Sure, looks like it's going to take a little while to fix up the Travis configuration anyway. I was really trying to fix just one thing (get screenshots working with the latest Firefox), but then had to do the rest just to get tests passing...I'll start with those things I had to fix along the way first. I'll keep this PR open for a while to test the overall Travis outcomes, if you don't mind.

[jphalip]

Sure, that's fine. You can also try setting up your own Travis project for more flexibility running the builds.

[jmbowman]

This code was split up into #64 and #65 .