Add implementation

Implemented a small widget using coffeescript. 

A demo is available in the `demo` dir. To compile a demo, instructions are available under the "development" section of the readme. You will have to do a `bundle install` to install dependencies, before compiling the demo.

In the demo, hover over the list elements in the sidebar and watch the markers being highlighted. Similarly, hover over the markers and the corresponding list element will be highlighted.