Be explicit about python2/pip2 in the install docs.

Ref #457.


OS X doesn't ship a python2 symlink

Also, from `--help`:

```
  -p PYTHON_EXE, --python=PYTHON_EXE
                        The Python interpreter to use, e.g.,
                        --python=python2.5 will use the python2.5 interpreter
                        to create the new environment.  The default is the
                        interpreter that virtualenv was installed with
                        (/usr/bin/python)
```