Simplify and bugfix setup.py
Install a specific App Engine SDK version via setup.py install

--install-appengine is no longer a boolean option but takes the version
number of the SDK to install. Same goes for the INSTALL_APPENGINE
environment variable.
Call appengine.install instead of shelling out
Pass setup.py install prefix on to appengine install

Also respects the setup.py install --user options and uses the user
site base dir if set, otherwise the given prefix or None if unset.
If not given build bindir from given prefix in _install