bump default Android API level to 19
Update proguard for new Libgdx versions
rebase of desktop proguard
Fix missing or duplicate assets

when running desktop/run assets are copied to desktop/target. This caused
assembly to report multiple assets. Without assets explicitly added, they
were missing from jar when built without previously running the code.
I've replaced task dependency with runtime:full-classpath, which triggers
both compilation and resources copying. Now, resources are always included
exactly once.
fixed handling of provided dependencies in assembly task
Merge pull request #72 from aginiewicz/updates

Updates
Merge pull request #61 from aginiewicz/proguard-desktop2

rebase of desktop proguard