Added command line options

Added 4 options;
-h -- prints help message
-b -- runs in batch mode so that no human interaction is necessary.
      It assumes that the answer would be yes to any question it would
      have asked
-f -- force install even in an non-empty directory
-P -- argument is the path to a valid python executable.  The use
      if $PYTHON is preserved, but overridden by this option.
That's true, but it's actually how I have used this sort of behavior in the past.  Usually you should only use a force flag if you know what you are doing, so I thought it wouldn't be bad to have such a thing.  I can remove it if you think it's too confusing.