Implement new HTML editor and output

This diff does three things:

1) It gets the Output tests working using Mocha and PhantomJS. You can run them using `gulp test`. See: f6969d0541f7658c0bb7429d4ee65b1c2be07508
2) It completely re-writes the output (previously: exec) internals. Everything is nicely relative and in clean Backbone views.
3) Implements a crude initial implementation of an HTML Ace Editor and an HTML Output. You can try this out in: `demos/simple/html.html`. See: 1449a3b019ed565f69edcf9f36014918f423b4cb

A lot of files were moved around in this diff, so hopefully it's readable.

Test Plan: All the tests pass and in my initial tinkering everything seems to be working as expected.
Presumably we'd have to move to some sort of package system (maybe a require-based thing?) in order for this to happen. Might be worth considering at some point!