Switch from Grunt to Gulp

ref #1110

* [ ] use variables for directory names
* [ ] update docs / posts on site
* [ ] re-think `assets/src/` and `assets/dist/` dirs?
* [ ] add `app.json` (see comments from @austinpray below)
* [ ] fix dependency issues
* [x] separate out minifying and renaming to separate prod only tasks
* [x] asset improvements (ref #1149 & `assets-improvement` branch)
This needs to go.