Support MacPorts-installed PyPy out of the box

Issue #40 on hold
Chris Jerdonek
created an issue

I tried installing PyPy for the first time to add a pypy test environment. I used MacPorts to install PyPy, but tox wasn't able to find it. It looks like MacPorts expects you to invoke PyPy using "pypy-c".

I'd like my tox configuration to be usable by everyone since it's part of a public repo, so I don't want to include something non-standard in the config file. What is the correct way to handle this? Perhaps tox can look for pypy in the pypy-c location on its own? Thanks.