Mark tests which fail on a Mac

So can do:
mocha -g "@notmac" --invert test/integration/
Link to correct Selenium downloads place
Update README for new Selenium

Plus explain that you have to unzip chromedriver.
Make test run on own
Simplify default user rules

Before, if defaultContext wasn't in canBeReally list it would
pick a random one. Now it just uses the user. This also simplifies
the overall logic.
Also check absence of "days left" message

On narrow screens, "Free trial:" doesn't show anyway, so we need
to check for something else.
Short free trial message on narrow screens

Partly because it overlaps and is unreadable, partly
because integration tests were croaking on this.
Remove @notmac
Merge pull request #639 from scraperwiki/fix-tests-osx

Fix tests osx