Update js minification process to use uglifier instead of jsmin

I made a couple small updates to get rid of jsmin and use uglifier instead. I tried using yui for js minification but it threw errors when testing locally with my coffeescript files. Using uglifier worked great with my coffeescript files. The uglifier gem seems to be actively maintained which is why I chose it for js minification. This commit should resolve issue #17.

I did not add any tests. The [sinatra-assetpack](https://github.com/rstacruz/sinatra-assetpack) library seems to have some good tests for their asset pipeline so perhaps we could use a similar approach for unit testing.