updated to python 2.7 & 64 bit libs for os x 10.9
support external editors

(by reloading from disk when a file has been modified while the app was
in the background)
auto-smartquotes in 10.9 were preventing creation of strings in the editor
zooming is now relative to the center rather than corner of graphic
empty output panel at start of each run
tidied up the zoom control
added command line tool to run scripts externally