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.
Btw., will this even work? I think with bash you need to say if [[ $batch_flag == "false" ]]; then... Same everywhere else.