One artifact per theme

This pull request add the generation of one webjar artifact per theme with all bootstrap resources correctly organized. It should solve issue #2 
Just use something like this in your project

<link rel="stylesheet" type="text/css" href="webjars/bootswatch-spacelab/3.0.3+1/css/bootstrap.min.css"/>
<script type="text/javascript" src="webjars/bootswatch-spacelab/3.0.3+1/js/bootstrap.min.js"></script>