Fix Modernizr and focus issues when embedded in about:accounts

* Modernizr blows up when trying to query for indexedDB. To get around this, for prod, we build modernizr with only the components we need. This has the advantage of being a smaller download as well. Since modernizr was throwing an exception, there were some interesting side effects - like the #stage element was never hidden in the embedded context. With modernizr working, content remained hidden because of the way $.show works, and this had to be fixed.
* Only elements that are visible can be focused. Keep trying to autofocus an element until it is focused.
* Ensure requirejs has a cache busting URL.

issue #384
fixes #353
Merge pull request #385 from mozilla/issue-353-modernizr

Fix Modernizr and focus issues when embedded in about:accounts