Support virtualenvs created with --python=/usr/bin/pythonX.Y rollup

[paul-krohn]

Supersedes #7 and #4. Updated the regular expression to make the version decimal optional, as it does not appear on some platforms.

[bukzor]

The shebang can also be $VIRTUAL_ENV/pypy or any number of things. The key bit is that the shebang points to a file within the original $VIRTUAL_ENV path, which we can extract from the activate scripts. See #10

[paul-krohn]

I gave up on this getting merged, and made Yet Another Fork.