Add Ability to Select Plot Line

1) Added delegate functions for handling down and up events while over the plot line.
2) Added delegate functions for handling the selection and deselection of the plot line.
3) Added delegate function for performing graphics operations just before drawing the plot line.
Speed up Symbol drawing
Added Rafał Wójcik to the license.
Updated the plot symbol clipping rectangle to account for shadows and the line width.
Merge branch 'release-2.0'
Documentation fixes.
Fixed a bug when converting floating point NAN values to NSDecimal.
Moved the Quartz framework into the Frameworks folder in the Mac Plot Gallery project.
Removed some type casts that are no longer needed.
Override -[CALayer display] to skip creating a drawing context when not needed.
Collapse the axis set layer to save memory if it is not needed to draw the plot area border.
Fixed the iOS hosting view so it only removes notifications that it added itself. Removing all notifications sometimes caused problems with subclasses. Fixed issue #81.
Fixed some static analyzer warnings.
Clear the isDragging flag on a pointing device up event. Coding style updates. Fixed issue #65.
Added Mike Rossetti to the license.
Changed the plot data line hit detection path to use Core Graphics for cross-platform support.
Renamed the scatter plot line selection delegate methods to standardize the naming scheme. Coding style updates. Fixed issue #23.
Added scatter plot data line selection demos to the Plot Gallery app.
Removed the data line was deselected delegate methods.
Fixed a memory leak.