Remove extraneous supersource as GWT now supports stripping of `@GwtIncompatible`-marked methods fields and classes.

Add a local GwtIncompatible annotation, and remove GwtCompatible.
Remove local GwtIncompatible and use GWTs since there's already a compile-time dep to build the -gwt artifact.
Merge pull request #98 from cgruber/removegwtemul

Remove extraneous supersource as GWT now supports stripping of `@GwtIncompatible