Watch style changes more intelligently

Avoid adding compiled files to the src folder.
Leverage grunt to watch for changes.
Support a quick watch task, so we can avoid full builds during development.

One can now run `grunt` or `grunt watch` during development to auto-recompile the scss files. Local API servers should point to `.../staging` instead of `.../src`. To build, use `grunt build` explicitly, since the default task has been changed.

No other build steps will take place for the staging task, so this should be on par with just using `sass --watch`. But now it fits better with the rest of our development flow and avoids cluttering the source tree/repo with compiled files.

Best to clean/reinstall node modules after merging:
```
rm -rf node_modules
npm install
```