wtperf stats go into a separate "wtperf" section in the sidebar.

Files of either type can be provided at the command line, even mixed in one call. The file type is determined when looking at the first line.

For example, this is a valid call to wtstats:

python wtstats.py ./WTPERF_RUN/monitor ../test/WiredTigerStat.fixture

Adding the wtperf if conditions all over the code is not very clean, I'd like to build two individual parser classes in the future and clean up that code.

This change would make the old wtperf_stats tool obsolete (entire tools/wtperf_stats/ folder could be removed incl. nvd3 references).