Improve dr-charts

[jiangpeng166]

1. because many projects have head file with name "ConStants.h", I rename it with new name "DrConstants.h"

(Mainly for MultiLineGraphView)
2. when I use this tool, I find it doesn't support negative number for yAxis, fix this issue
3. Add three more methods for protocol MultiLineGraphViewDataSource

• (NSMutableArray *)dataForXAxisForAllLines;
  User can define the xAxis Value Array by self.
  e.g. (a line is {1,2,3}, the other is {6,7,8}, user can define the xAxis Value Array using the algorithm neeed)

• (NSString *)customYAxisFormat:(float)yAxisValue;

• (NSString *)customXAxisFormat:(NSString *)xAxisValue;
  User can define the formant for Axis value by self.

4. Add more property to define the chart more flexible
   //Set step number for xAxis display value
   @Property (nonatomic) int xAixsStepNumber;
   //Set step number for yAxis display value
   @Property (nonatomic) int yAixsStepNumber;
   //Set animation duration time
   @Property (nonatomic) float animationDuration;
   //Set default min value for yAxis
   @Property (nonatomic) float yAixsDefaultMin;
   //Set default max value for yAxis
   @Property (nonatomic) float yAixsDefaultMax;
   //Set axis Line color (not same with gridLineColor)
   @Property (nonatomic, strong) UIColor *axisLineColor;