Prevent duplicate assets

Refs #182.

This PR removes the internal map of loaded scripts and styles in favor of looking them up when needed. This adds a slight overhead, but will not miss scripts that were present on page load or scripts that were loaded outside of F2.

This also makes a minor change by removing the `test` task from the end of the `js` task. I had previously added it to make sure we weren't committing any breaking builds, but our tests are currently too slow to support this workflow. I'm looking at making the tests super speedy for 1.5.