Replace SASS with assets from Ghost-UI

closes #2368
- Remove Ruby dependency (Gemfiles)
- Remove sass/fonts from core/client/assets/
- Add copy task to Gruntfile to copy over ghost-ui assets
- Install Ghost-UI via bower
- Changed .npmignore/.gitignore
- Remove grunt-contrib-sass dependency
- Remove mentions of Ruby in CONTRIBUTING.md
Merge pull request #2369 from halfdan/bower_all_the_things

Replace SASS with assets from Ghost-UI