Fixed colorbars, which have not been appearing in 2d plots...
New attributes colobar_label, colorbar_ticks, etc. were added but omitted from 2d_pcolor plot_type, for example.  Added them more generally.
Also modified frametools.py to check if attribute add_colorbar exists rather than throwing an exception that was being ignored.
Merge pull request #85 from rjleveque/fix_colorbar

Fixed colorbars, which have not been appearing in 2d plots...