Fix shadow cropping + add ability to disable shadow

Fixed shadow cropping problems by removing shadow from drawRect and using CALayer instead (requires import of QuartzCore.h). 
![shadow-fix](https://f.cloud.github.com/assets/1586504/120712/95d7e240-6d23-11e2-8ff6-794d7a8ab177.png)

Added hasShadow BOOL property (part of issue #28).