Remove uses-sdk attribute (overriden from build.gradle)

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>
Depend on Android support library 20.0.0

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>

Conflicts:
	library/build.gradle
Merge pull request #34 from kontalk/upstream

Upgrade support library version and other manifest fixes
Merge conflict with previous commits