Compatibility updates

I've made some minor changes to this plugin to fix compatibility issues with Atmosphere 2.0.3 and above.
I've also made changes to make this compatible with the Asset Pipeline plugin for the Atmosphere JavaScript files.

I've updated the the dependencies to include Atmosphere 2.1.6 and it also includes Atmosphere JavaScript 2.2.1.

It appears you no longer actively maintain this plugin, if so I would be happy to take it over. As it currently stands the plugin on the Grails website (http://grails.org/plugin/atmosphere) does not work out of the box with the Grails 2.4 default configuration. It would be good to fix this.