Updated for python 3.x print() syntax

print() command syntax was obsolete, and wouldn't run with python 3.x
Added usage information

Previous version didn't display usage information correctly at runtime,
it didn't actually show what the 3 mandatory arguments needed to be and
in which order.
Merge pull request #1 from g-a-c/master

Updated to run with Python 3.x and improved usage information at runtime