Fixes in GWT backend:
-added log configuration with GwtApplicationConfiguration.log
-more accurate value of GwtGraphics.getDensity()
-Skin emulation takes NinePatch padding into account (copy-paste from not emulated class)
-fixed filtering of asset files in PreloaderBundleGenerator
backgroundFPS should be 0 by default.
Added setWriter method.
Merge remote-tracking branch 'origin/master'
Getter for writer.
cancel method.
Add HttpParametersUtils to GWT emulation.

Because URLEncoder does not exist in GWT-land, the emulated class uses an
equivalent encode method provided by GWT's URL class.
Merge pull request #288 from kreymerman/gwt_fixes

Couple of small fixes in GWT backend
Fix GwtGraphics.getDensity()

Previous commit changed to a double, which caused a type mismatch with the return type.