Set the PYTHONPATH in the script

Bash doesn't inherit environment variables, so setting it in Popen is
pointless.
Add a test for the help output of activate and deactivate
Add more tests for activate/deactivate help
Python 3 support is not experimental. Python 3.2 is not supported.
Fix activate tests to work

The tests can't use bin/conda, as it uses #!/usr/bin/env python, which doesn't
work so well when you remove the root python from the PATH. So instead we
write out a custom conda entry-point, which has a hard-coded shebang line,
which is what a normal conda from an installation would look like anyway. Note
that if someone does use a conda entry-point like that, they will have
issues, but that should only happen if they setup.py install without
setuptools.