Added profiling directory to gitignore.
Removed unnecessary call to redraw in the matplotlib image viewer's
method for refreshing the image colormap.  Profiling while tabbing 
showed the CPU time spent in the refresh colormap method dropped
from about 30% to about 3%.
Added optional redraw parameter to view_image and draw_circle.
This allows us to only redraw the figure once when moving to look at a 
new image with circle draw on it.
Profiling showed that as a result the time spent in the view_image
method when tabbing was reduced from about 40% to about 10%.
Merge pull request #88 from drusk/optimize_tabbing

Closes #86.