Remove popoverEdge from the initializer

It’s already an assignable property
Move the origin rect to a property
Allow the background view to be set instead of the class
Use instance methods instead of class methods
Make the arrow size customizable
Use C99 struct initialiser syntax
Make popoverEdge and popoverOrigin readonly externally
Clear the clippingPath on dealloc
Keep the clippingView on top
Add a note about the memory management
Pass the background view into the initializer
Merge pull request #120 from github/popover-view-customization

Make RBLPopover's view customizable