Change main DownloadBuilder API, remove themegallery method, added buildThemesBundle

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 )
- callback( err, bundleFiles ): bundleFiles is an Array of { path:<path>, data:<data> }'s.

Generates the theme bundle with base and all themes from themegallery.