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.
What will it take for us to be able to move away from sticking stuff straight on window?