Make absolute forgetten-relative-filepaths
Change main DownloadBuilder, from Frontend to Main

Previously, main was `frontend.js`, now it is `main.js`. Frontend can now be accessed by `download.frontend`, where `download = require( "download.jqueryui.com" )()`;

The new API:

Main( options ) -- or require( "download.jqueryui.com" )( options )
- options [ Object ]: key-value pairs detailed below.

options
- host [ String ]: optional, specify the host where download.jqueryui.com server is running. Default: "" (empty string).
- config [ Object ]: optional, if present used instead of the `config.json` file;

attributes
- frontend [ Object ]: for more details see `frontend.js`.

Main#buildThemesBundle( callback ), generates the theme bundle with base and all themes from themegallery.
- callback( err, bundleFiles ): bundleFiles is an Array of { path:<path>, data:<data> }'s.

On the config, a prepared path can be specified for stable or legacy by using the `path` attribute, instead of passing the version plus letting `grunt prepare` prepare it. Eg.

download = require( "download.jqueryui.com" )({
   config: {
       "jqueryUi": {
           "stable": { "path": "my/stable/path" }
       }
   }
});

The above prevents jQuery UI of some workarounds on its grunt `release_themes` task.
Fix stripReleasePath. It wasn't returning relative paths when used as an external lib
Export method for generating themes from a given version and folder. Fixes #106