Here is a patch which changes get_scmtool to raise nose.SkipTest instead
of an ImportError when instantiating a scmtool class.

With this patch, I can run the tests on my amd64 machine which I was not
able to get P4Python installed on (I've sent patches off to perforce),
and I get 5 tests skipped rather than failed, a much nicer result.

I think this means that people writing an SCMTool should be careful to
allow their module to be imported even if the SCMTool is not installed,
and only do any tool-specific imports in __init__. The existing tools
are written that way at least, and I think it is a good approach.

Another option that has been mentioned is to trap the import errors in the tests themselves. The desire is that nose not be required for deployment, only for testing, and that is a good point.

Ran manage.py test.