Initial work on running automated UI tests on many browsers, driven by browsers.json, using BrowserStack.
Turns out learn.code.org has jquery.simulate already available, no need to inject.  Also updated browser list.
Merge pull request #286 from code-dot-org/automate

More BrowserStack automation work
Merge pull request #291 from nanan9177/blockly_fix

Quick fix for activities with nil level_source_id.
Remove banner image from student/teacher signup.
Merge branch 'master' of https://github.com/code-dot-org/dashboard
Move browser rake task to be only in development. Later, still need to do test on staging.