Warn when missing 'webtest' package (needed for tests)

webtest isn't in requirements.txt because it's only required to run the
tests. This commit will make run_tests.py try to import it and fail with
a helpful error message in case it fails.
Also warn when trying to run tests while missing nose

Just in case someone hasn't got nose installed either.
Merge pull request #121 from zoni/webtest

Warn when missing 'webtest' package (needed for tests). thx