Updated deployment target to 10.8 and project settings to those recommended by Xcode 5.

PLTabViewController now uses the simpler CGColor method of NSColor to convert the tab background color (this was available in 10.8).
Configured all xib files to open with Xcode 5 and build for the deployment target.

Needed to update the constraints for PLEditorViewController. Also updated an issue in PLScroller coordinateScalingFactor method that could result in returning NaN (this error appeared while updating the constraints in PLEditorViewController).