Fixed and simplified rounded arrow code.

Fixes issue #28. The problem was related to drawing the rounded arrow points in the incorrect order.

Also:
- Simplified arrow drawing code: no longer creating needless `UIBezierPath`s, using the single `outerPathRef`instead.
- Made all rounded arrows equal (up/down arrows were different than left/right ones)
- Generalised target point calculation for rounded and straight arrows. 
- Renamed `adjustsTintColor` to `dimsBackgroundViewsTintColor`.
- Changed `usesRoundedArrow` and `dimsBackgroundViewsTintColor` from `NSNumber` to `BOOL`. We avoid packing/unpacking between `NSNumber` and `BOOL` when setting this.
- Updated README.md.