ENH: Add 'describe extension' to SlicerWizard.py

Add an action to "describe" an extension (i.e. dump its generated
description content to stdout). This is useful both as a user check
before publishing, and potentially can be used by CMake as an alternate
means of generating the s4ext file for the build.

Note that at the moment the extension must have a git repository.
Eventually we want to support subversion repositories and also
extensions that don't yet have a repository, but a number of places will
need to change for that (e.g. publishing should also support svn).
STYLE: Rename Slicer(Wizard.py) -> Extension

Given its current functionality, SlicerWizard.py is more appropriately
the Extension Wizard; rename it accordingly. This also allows us to
shorten the actions (by dropping 'extension' from the end).

Also, the 'submit' action has been renamed to 'contribute'.

Note that the module code has NOT been moved, as it conceivably could be
used by some other script. (Although the SlicerWizard class is renamed.)
Thus, the driver script now says 'from SlicerWizard import
ExtensionWizard'.