Added some error checking to catch corrupted input (prints a warning and
skips the line)
Add a comment about the potential for the age value to be negative
Changed the list we used to keep track of nodes during a traverse to a
deque.  This sped things up substantially because list.pop() is so
expensive.  Thanks to Devesh for pointing this out.

Also added some code to check the interpreter version before running.
We want at least v2.6 so that the exception classes have the message
parameter.  (Some of the CCS RHEL-5 servers default to python 2.4.3.)