Updated GWT backend to GWT 2.6.0

- Java 7 support (yay!)
- IE 6/7 support disabled by default.
- See GWT changelog for more
- Pulled from maven central
Merge remote-tracking branch 'origin/master' into gwt-2.6.0
Updated Changes to reflect gwt upgrades.
Merge remote-tracking branch 'origin/master' into gwt-2.6.0
Fixed merge conflict
fixed merge conflicts
Merge branch 'master' of https://github.com/libgdx/libgdx
Merge branch 'master' of https://github.com/libgdx/libgdx