Bug #3305 - Have embed.js load Popcorn via require

This pull request does a bunch of cool things. The primary goal was to have RequireJS load Popcorn, and as a side-effect we get:

1) Nuke the whole concept of external assets loaded by the server being different from the JS, eliminating a whole class of "which file do I edit to get my new plugin loaded"
2) IE9 shims will *always* load before app code
3) We only load a single JS file now, so faster loading times
4) The r.js optimizer will concat all of the Popcorn files into `embed.js` so that we'll eventually be able to remove all of our build-related code for that

:yum: 

This is only the first part; I'll do a similar thing for butter.js in another patch.