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
Cheers for the catch, we'll bump this to latest (though this is our production processor server now: https://github.com/jsbin/pennyworth - Stylus is being added, @allouis is just working on getting tests in place).