Fixes #1002

This is related to my fix in #658. Apparently, MacOS adds a git
executable that is just a stub which displays an error. This gets past
the try/except I added in #658, and allows all of the GitPython
components to be imported, but since the executable is not *actually*
git, it results in an exception when ``refresh()`` attemepts to run a
``git version``.

This catches any raise of one of the custom exceptions defined in
`git.exc` during the imports in the dunder init, and raises an
`ImportError` in those cases.

Added additional information in the import warning/error that tells the
user how to silence the warning/error. Also added a GIT_OK variable
that allows for a quick check whether the refresh has succeeded instead
of needing to test an actual git command.

Renamed to simplify and avoid issue with nose tests trying to use
`setup` as a setup for testing. Unittest implements basic test for
refreshing with a bad git path versus a good git path.

Discovered that the remote module also relies on the git executable as
such it also needs to be “refreshed” anytime the git executable is
updated or changed. This was best solved by moving the setup function
into the top level __init__ where the setup simply calls
git.cmd.Git.refresh and git.remote.FetchInfo.refresh.