Added a hard anchor point

Added a hard anchor point that will allow the sliding panel to stop at
the hard anchor point no matter what. Currently the user can slide past the anchor point but
it will slide to the hard anchor point when the view is released.

To set the hard anchor point in the MainActivity the user must call "layout.setHardAnchorPoint(true)".
This will override any anchor point set with "layout.setAnchorPoint(<float>)"
The hard point is calculated on line 523

The sliding panel can be dragged pass the hard anchor point but when the view is released, the sliding panel will smooth slide back to the hard anchor point. I do not think that this can be stopped since technically the panel is sliding to 0. I may be wrong.

I have also added the ability to turn off the cover color that is drawn on top of the main panel when the sliding panel is expaned. This was something that I was asked to do so I left it in.

I also added back some of Android margin and padding calculations that were missing from SlidingPanelLayout.class