Allow setting an explicit handle height

[timbodeit]

I am using WKVerticalScrollBar with a round button as the scroll handle like so:

To allow setting custom handle dimensions, I added an explicitHandleHeight and useExplicitHandleHeight attribute to WKVerticalScrollBar.

Other changes included in this pull request:

• Immediatly update the handle color and handle width when setting them. Previously these changes only took effect after dragging the handle for the first time.
• Explicitly specify WKVerticalScrollBar as a non-ARC project in the podspec.