canvastools subpackage for scikit-image viewer

[Steven Silvester]

Tony,

I'm going all in on this with you.  I have been hacking at matplotlib recently as well, and have demonstrated the following in PySide and PyQt4:
- right click menu with grid toggle and colormap selection, and simplified uimenu creation
- roi with stats overlay (mean, std, min, max)
- cross section slice view
- scroll zoom
- improved colorbar placement

I also wrote a tool in enaml that allows you to pick a skimage package and function from dropdowns. It then displays the docstring and allows you to write the function call inline, and safely evaluates it. 

Thus far, it has all been proof of concept and for use at work, but I'd like to start integrating these features into your viewer. 

I will start a repo on my github to demonstrate the features prior to integration. 

Thoughts?