Added pie chart property to hide label under a user defined limit

I added this property because showing all labels, when there are a lot of small items in the dataset, is quite confusing.

Example:
piechart.labelPercentageCutoff = 0.05
hide all labels with value less than 5% (I use this most of the times).

@@ -38,6 +38,9 @@
 /** Show absolute values not relative i.e. percentages */
 @property (nonatomic) BOOL showAbsoluteValues;
 
+/** Hide percentage labels less than cutoff value */
+@property (nonatomic, assign) CGFloat labelPercentageCutoff;
+
 /** Default YES. */
 @property (nonatomic) BOOL shouldHighlightSectorOnTouch;