Generating readable graphs for large clos topologies; changed Entity::ComputeRecentlySeen to be more flexible

-Changed bandwidth usage gnuplot script to better display results for large graphs; the y-axis is now in units of KB/sec
-Log statements of level WARNING, ERROR, and FATAL are output to stderr; log statements of level INFO are output to logs/
-Added random jitter to first heartbeat initiation
-A node is now considered recently seen if its heartbeats were seen at least m times in the last n seconds.  Changed Entity::ComputeRecentlySeen to reflect this.
-Added a script to turn Clos topologies generated by Murphy's graph tools into the format understood by the reader (note that this is in addition to some changes I made to urphy's convert.py but the change is so minor I feel it would be silly to start a non-local copy of Murphy's repo)
-Added pdf and log files to .gitignore