enhanced support for shell variables in the python_interpreter setting

use os.path.expanduser and os.path.expandvars to support python_interpreter settings such as:

- "~/.virtualenv/whatever/bin/python"
- "$USER/.virtualenv/whatever/bin/python"
- "$VIRTUAL_ENV/.virtualenv/whatever/bin/python"
- "${whatever}/.virtualenv/whatever/bin/python"
Merge pull request #177 from NorthIsUp/patch-1

enhanced support for shell variables in the python_interpreter setting