UIControlEvents, Fill with alpha

Fixes #13, @hebertialmeida's #15, #16 .

• Support for UIControlEvents, fired in the same order as with UISlider
  Add listener:

  self.discreteSlider.addTarget(self, action: "touchUpInside:event:", forControlEvents: .TouchUpInside)
  

  Listen:

  func touchUpInside(sender: UIControl, event:UIEvent) {
      print("touchUpInside")
  }
  

• Use CGContextSetFillColorWithColor to support alpha channel