Sass client support + promise based processors

- [ ] Ready for merge

Also re-implement client side stylus (ref #902).

Also optimised the render process, so if the source hasn't changed, then the compile process is ignored.

Todo:

- [x] Server side support
- [x] Reinstate stylus server side
- [x] Ensure processors run in `childProcess.fork` to protect parent
- [x] Fixes #1495
- [x] Test local install (without zeromq)
- [ ] Fade "Convert to CSS" out unless a non-CSS language is selected. Fixes #1511
- [x] Investigate `@import` support
- [ ] Show errors in panel like js
- [ ] Rename `port` in `address`
- [ ] UI for language dropdown menu
- [ ] Debounce the call to the processor
- [ ] Syntax highlighting in CodeMirror (not sure if already there)

Nice to have
- [ ] Possible UI for adding libraries/frameworks
Regarding Stylus: yay! But why not the latest version? We're maintaining client-side build for the latest versions, you can grab it and try it from [there](https://github.com/LearnBoost/stylus/blob/gh-pages/try/stylus.min.js) (used at [this page](http://learnboost.github.io/stylus/try.html) for now), for example.

Also, if you'd happen to see any problems with Stylus, feel free to open issues, we'll try to fix them ASAP, as having Stylus at jsbin would be really cool :)