Default backgroundColor to clearColor (issue #49)

When initializing in IB, it's clear.
But initWithFrame set it as nil - and combined with the default opaque=YES - it was filling the background with black.

Charts/Classes/Charts/ChartViewBase.swift

@@ -116,6 +116,7 @@ public class ChartViewBase: UIView, ChartAnimatorDelegate
     public override init(frame: CGRect)
     {
         super.init(frame: frame);
+        self.backgroundColor = UIColor.clearColor();
         initialize();
     }