Line 239 typo, layout_gravity to gravity

Trying to implement this library earlier I noticed i was receiving an error "layout_gravity must be set to either top or bottom" and when I set the layout_gravity to bottom noticed that I was still receiving the error and the app was still crashing out on initialization. When I went to the repo I noticed that the android:gravity was set to bottom not the android:layout_gravity. So I changed that and everything worked. So  I have changed the thrown IllegalArgumentException to say gravity instead of layout_gravity.
Added in panelIsOverlay boolean from my issue

Added in the panelIsOverlay boolean for the Show/hide jumpy issue that I was having.
Added in the ability to make overlay in xml

Added the java code to make overlay.
Added overlay attr

Added in the overlay attr.
Merge pull request #144 from adfleshner/master

 Overlay attribute added as well as Line 239 typo, layout_gravity to gravity