changes to kernel.py to support plotting with Octave-snap 5.2.0 1) Added a plot settings 'plot_dir' for where to create temporary directories 	(defaults to None) 2) Changed _get_executable() to detect 'snap' in the path to Octave and leave 	the user set OCTAVE_EXECUTABLE as the executable
changes to kernel.py to support plotting with Octave-snap 5.2.0
1) Added a plot settings 'plot_dir' for where to create temporary directories
	(defaults to None)
2) Changed _get_executable() to detect 'snap' in the path to Octave and leave
	the user set OCTAVE_EXECUTABLE as the executable

Calysto/octave_kernel,Calysto/octave_kernel