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 point calculation for rounded and straight arrows. 
- Renamed 
- Changed `usesRoundedArrow` and `dimBackgroundViewsTintColor` (previously adjustsTintColor) from `NSNumber` to `BOOL`. No need to use `NSNumber` for simple `BOOL` properties.
- Updated README.md