`python.py` steals control in `cmd.exe`

at least it does on windows 7 box

steps to reproduce in cmd.exe:
- clone latest master of rvirtualenv
- `C:\rvirtualenv> rvirtualenv.py NEWENV`
- `C:\rvirtualenv> NEWENV\bin\activate.bat`
- `C:\rvirtualenv> python`

now you can see python prompt, but it is not interactive (like running in the background)

probably somehow related to #2
