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.
Is this the intended behavior, to just run in a non-empty directory irrespective of what was in there? If there is an incomplete (== failed) installation in the directory, the forced install could fail in mysterious ways.