Additional RSColorPickerViewDelegate methods

I added optional RSColorPickerViewDelegate methods for more detailed feedback during user interaction.
I have an RSColorPickerView in a UIScrollView and wanted the ability to disable scrolling when the user was actively using the color picker, and enable scrolling when the user was done using the color picker.

The new delegate methods are:

```obj-c
- (void)colorPicker:(RSColorPickerView*)colorPicker touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event;
- (void)colorPicker:(RSColorPickerView*)colorPicker touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event;
```