customizable app root

This PR updates the Singularity UI to generate paths based off of SingularityConfiguration values instead of using hardcoded paths.

This is accomplished by:
 - Rendering `index.html` using mustache template (instead of a static file), allowing us to pass through the static assets path.
 - serve `config.js`, which provides the app root, api root (i.e. `/singularity/v1/`), and mesos slave port(s) based off of values from SingularityConfiguration
 - remove `SingularityBrunchServlet` in favor of using `dropwizard-views`

One downside is that in order to make this all work, I had to prepend all the API `@Path`s with `/v1`. Kind of a bummer but I don't think it's the end of the world.

@wsorenson @timmfin @gchomatas