MEP22 Navigation toolbar coexistence

This version of the MEP22 implementation https://github.com/matplotlib/matplotlib/wiki/Mep22#implementation 

The proposed solution is to take the actions out of the `Toolbar` and the shortcuts out of the `Canvas`. This actions and shortcuts will be in the form of `Tools`.

A new class `Navigation` is the bridge between the events from the `Canvas` and `Toolbar` and redirect them to the appropriate `Tool`.

@OceanWolf  I mean http://matplotlib.org/faq/installing_faq.html#source-install-from-git 
For the initial setup and http://matplotlib.org/devel/gitwash/development_workflow.html for the workflow.