Fix date format issues, form error handling

Define DATE_DISPLAY_FORMAT in models.Event to use for parsing and
formatting displayed dates, and configure datetimepicker to use an
identical format (though it uses a different syntax).

Refactor event updating/creating to allow for in-form error reporting,
in place of just ``res.send(400, errors)``.

Add an event.getEventUrl() function that returns the shortName URL (if
present), or the ID-based URL.  Update event redirect handlers to use
this.

Add a new express.locals var, "baseUrl", to use in place of hard-coded
``unhangout.media.mit.edu``.

Remove unnecessary _.bind calls from lib/unhangout-routes.js.

Update tests to reflect these changes, and add an initial .travis.yml
file.