Stylus support

Turns out LESS is becoming *less* and *less* maintained (and is hella slow). Also, turns out Stylus is amazing (and is hella fast).

* [LESS](https://github.com/cloudhead/less.js) requires node.js.
* [SASS](https://github.com/nex3/sass) requires Ruby.
* [Stylus](https://github.com/LearnBoost/stylus) requires node.js.

Some of its niceties:
 
* [mixins with default values](http://learnboost.github.com/stylus/docs/mixins.html)
* [keyword arguments](http://learnboost.github.com/stylus/docs/kwargs.html)
* [rest... arguments](http://learnboost.github.com/stylus/docs/vargs.html)
* [automatic vendor prefixing (if we want)](http://learnboost.github.com/stylus/docs/keyframes.html)
* so much more…

Oh, and did I mention [automatic recompilation out of the box](http://learnboost.github.com/stylus/docs/middleware.html)?

How is it syntactical different from LESS?
* Nesting format stays the same. Still use curly braces.
* For variables instead of `@` use `$`.
* For mixins instead of `.mixin()` write `mixin()`.
* Instead of overloading the mixins for optional arguments, use default argument values.

Blockers:
* https://github.com/jsocol/jingo-minify/pull/42
* IT to use `npm install` (I've added `stylus` to our `package.json`)

I did a visual comparison, and did a `diff` of the files and they're almost nearly identical, save some rounding for `fade-out` and `darken` calculations.

I'd like to do this with AMO too - I've got a [script](https://gist.github.com/cvan/5061790) so the conversion is pretty trivial.

Using stylus shaves quite a few seconds off `compress_assets`:

    python manage.py compress_assets --settings=settings_local_mkt  16.00s user 2.34s system 99% cpu 18.420 total
    python manage.py compress_assets --settings=settings_local_mkt  26.14s user 3.84s system 99% cpu 30.095 total

And the file-size differences:

    less/:
    -rw-r--r--   1 chris  wheel   82191 Mar  2 18:03 consumer-all.css
    -rw-r--r--   1 chris  wheel  104768 Mar  2 18:03 consumer-desktop-all.css
    -rw-r--r--   1 chris  wheel  123795 Mar  2 18:03 devreg-all.css
    -rw-r--r--   1 chris  wheel   70614 Mar  2 18:03 ecosystem-all.css
    -rw-r--r--   1 chris  wheel   22132 Mar  2 18:03 in-app-payments-all.css
    -rw-r--r--   1 chris  wheel    6682 Mar  2 18:03 lookup-all.css
    -rw-r--r--   1 chris  wheel   29934 Mar  2 18:03 offline-all.css
    -rw-r--r--   1 chris  wheel   84749 Mar  2 18:03 reviewers-all.css
    -rw-r--r--   1 chris  wheel   16344 Mar  2 18:03 splash-all.css
    -rw-r--r--   1 chris  wheel   33854 Mar  2 18:03 stats-all.css
    -rw-r--r--   1 chris  wheel    5455 Mar  2 18:03 themes-all.css

    styl/:
    -rw-r--r--   1 chris  wheel   76288 Mar  2 18:01 consumer-all.css
    -rw-r--r--   1 chris  wheel   98934 Mar  2 18:01 consumer-desktop-all.css
    -rw-r--r--   1 chris  wheel  121588 Mar  2 18:01 devreg-all.css
    -rw-r--r--   1 chris  wheel   58383 Mar  2 18:01 ecosystem-all.css
    -rw-r--r--   1 chris  wheel   16357 Mar  2 18:01 in-app-payments-all.css
    -rw-r--r--   1 chris  wheel    6637 Mar  2 18:01 lookup-all.css
    -rw-r--r--   1 chris  wheel   25061 Mar  2 18:01 offline-all.css
    -rw-r--r--   1 chris  wheel   83728 Mar  2 18:01 reviewers-all.css
    -rw-r--r--   1 chris  wheel   16551 Mar  2 18:01 splash-all.css
    -rw-r--r--   1 chris  wheel   33693 Mar  2 18:01 stats-all.css
    -rw-r--r--   1 chris  wheel    5426 Mar  2 18:01 themes-all.css

The main benefit as I see it to using Stylus is the rapid compilation, which makes this a great candidate for Fireplace. This streamlines our development and deployment for Marketplace as a packaged app.