Updated GWT/HTML5 Backend to 2.6.0

- Java 7 Support (yay!)
- Jars pulled from maven central ( http://search.maven.org/#search%7Cga%7C1%7Cg%3A%22com.google.gwt%22 )
- IE 6/7 support disabled because of upgrade. It can be enabled, but it is going to be removed in the next major release
- More changes on GWT changelog

Tested on:
Firefox 26 on Linux (x64). Ran several tests and Superjumper. 
More testing is appreciated as there is much that needs to be covered and the tests only do so much.