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`.

Actually for setting the fork, is 
http://matplotlib.org/devel/gitwash/forking_hell.html#forking
http://matplotlib.org/devel/gitwash/set_up_fork.html#set-up-fork

Then you go to the installation, and then to the workflow.