Fix ConcurrentModificationException

Occurs when multiple listeners are registered.

Signed-off-by: Greg Meiste <greg.meiste@gmail.com>
Compile for and target API 20. Use Support Library r20. Closes #121

Google must have fixed the SwipeRefreshLayout jitter in r20 of the
support library, because the issue is no longer seen.

Signed-off-by: Greg Meiste <greg.meiste@gmail.com>