Rectangular shape support

If you try to use PWProgressView with a non square frame, it breaks the layout. In fact, I see that you override setFrame to force height and width to be the same. 

I've changed some geometry calculations so it behaves exactly the same when using a square frame, but allows to use a rectangular frame. Progress circle sticks to the center (vertically and horizontally) , and gets resized as always depending on PWCenterHoleInsetRatio constant.

Regards