Add the option to use minified js files.

useMinFiles added as an optional configuration value, defaults to false.  When true, the main js file(s) defined in the bower.json are checked to see if a '.min.js' file also exists and use it instead.  This allows the use of pre-minified components which is preferred over end user mangling each one. If true, but the min.js file is not found it will preserve and concat the .js file.