feat(PieChart): redraw chart if layout changed

This is to support autolayout and constraints.

Resize layers for current bounds on layoutSubviews.
For that to work we need to have radiuses recomputed on each draw.

As a bonus you can now orverride `recompute` method to change
inner and outer radiuses. With possibility to change other attributes
in a subclass.

@@ -44,8 +44,16 @@
 /** Default YES. */
 @property (nonatomic) BOOL shouldHighlightSectorOnTouch;
 
+/** Current outer radius. Override recompute() to change this. **/
+@property (nonatomic) CGFloat outerCircleRadius;
+
+/** Current inner radius. Override recompute() to change this. **/
+@property (nonatomic) CGFloat innerCircleRadius;
+
 @property (nonatomic, weak) id<PNChartDelegate> delegate;
 
 - (void)strokeChart;
 
+- (void)recompute;
+
 @end

@@ -15,9 +15,6 @@
 @property (nonatomic) NSArray *items;
 @property (nonatomic) NSArray *endPercentages;
 
-@property (nonatomic) CGFloat outerCircleRadius;
-@property (nonatomic) CGFloat innerCircleRadius;
-
 @property (nonatomic) UIView         *contentView;
 @property (nonatomic) CAShapeLayer   *pieLayer;
 @property (nonatomic) NSMutableArray *descriptionLabels;
@@ -48,9 +45,6 @@
     self = [self initWithFrame:frame];
     if(self){
         _items = [NSArray arrayWithArray:items];
-        _outerCircleRadius  = CGRectGetWidth(self.bounds) / 2;
-        _innerCircleRadius  = CGRectGetWidth(self.bounds) / 6;
-        
         _descriptionTextColor = [UIColor whiteColor];
         _descriptionTextFont  = [UIFont fontWithName:@"Avenir-Medium" size:18.0];
         _descriptionTextShadowColor  = [[UIColor blackColor] colorWithAlphaComponent:0.4];
@@ -86,12 +80,20 @@
     
     _pieLayer = [CAShapeLayer layer];
     [_contentView.layer addSublayer:_pieLayer];
+
+}
+
+/** Override this to change how inner attributes are computed. **/
+- (void)recompute {
+    self.outerCircleRadius = CGRectGetWidth(self.bounds) / 2;
+    self.innerCircleRadius = CGRectGetWidth(self.bounds) / 6;
 }
 
 #pragma mark -
 
 - (void)strokeChart{
     [self loadDefault];
+    [self recompute];
     
     PNPieChartDataItem *currentItem;
     for (int i = 0; i < _items.count; i++) {
@@ -443,4 +445,11 @@
     [squareImageView setFrame:CGRectMake(originX, originY, size, size)];
     return squareImageView;
 }
+
+/* Redraw the chart on autolayout */
+-(void)layoutSubviews {
+    [super layoutSubviews];
+    [self strokeChart];
+}
+
 @end