RF: allow command runner to also set path and test

The command runner did not take into account that we'd have some scritps
to test.  Add ability to add script directory to path, and make some
tests.
BF: add pass of script path to sdist_tests

I wasn't passing the script path and therefore not collecting the newly
installed scripts.

The scripts need the package to be on the python path for their
subshells, so we also need to set PYTHONPATH for script calls.
RF: allow passing of label and doctests to testers

Useful if we want to run a subset of the tests.
Merge pull request #149 from matthew-brett/testers-with-scripts

RF: allow command runner to also set path and test

The command runner did not take into account that we'd have some scripts
to test. Add ability to add script directory to path, and make some
tests.