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.
I'd rather not add the "... using Python at $PYTHON" bit -- it will confuse the casual user who may expect we'll be using Anaconda python here.

Instead, I'd add an if clause _before_ this one such as:

    if [[ "$PYTHON" != "/usr/bin/python ]]; then
        echo "Using python at $PYTHON to install EUPS"
    fi

(the logic being that this will show up only if the user did something out of the ordinary).