set/reset title of shell window on Windows (issue #591)

This pull request defines an environment variable `VIRTUALENV_OLDTITLE`, which allows to change the window title of the commandline window in Windows. If set, `activate.bat` will use the `title` built-in command to set the title to a generic value, and `deactivate.bat` will "reset" it to the value of the variable.

Furthermore, I cleaned up the whitespace in the affected files.

I hope the additions to `.gitignore` are easily agreed on; `*~` is for backup files some editors create, e.g. `vim`.