It should not require a JavaScript runtime

- [x] find a good solution for seperating js from ruby
- [x] create simple angularjs app with grunt/requirejs and friends

After fresh install:

```
lucassus@lucassus-VirtualBox ~ $ mongo_browser 
/var/lib/gems/1.9.1/gems/execjs-1.4.0/lib/execjs/runtimes.rb:51:in `autodetect': Could not find a JavaScript runtime. See https://github.com/sstephenson/execjs for a list of available runtimes. (ExecJS::RuntimeUnavailable)
	from /var/lib/gems/1.9.1/gems/execjs-1.4.0/lib/execjs.rb:5:in `<module:ExecJS>'
	from /var/lib/gems/1.9.1/gems/execjs-1.4.0/lib/execjs.rb:4:in `<top (required)>'
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /var/lib/gems/1.9.1/gems/coffee-script-2.2.0/lib/coffee_script.rb:1:in `<top (required)>'
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /var/lib/gems/1.9.1/gems/mongo_browser-0.1.2/lib/mongo_browser.rb:9:in `<top (required)>'
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /var/lib/gems/1.9.1/gems/mongo_browser-0.1.2/bin/mongo_browser:5:in `<top (required)>'
	from /usr/local/bin/mongo_browser:23:in `load'
	from /usr/local/bin/mongo_browser:23:in `<main>'
```

## separate front end from the backend (sinatra app)
- [x] move all front-end related files to the separate folder
- [x] get rid of sprockets
- [ ] create grunt build file
- [ ] build production release to `./public` directory
- [ ] rename `lib` to `backend`
- [ ] do not include all files in the gem
- [ ] update Readme