Wtstats changes

@michaelcahill Please review these changes for wtstats.  Here's what I added:
- support that if you give it a directory it looks for all WiredTigerStat files in the directory, or uses whatever file you give it.
- added --all arg that will generate HTML files for the overall stats (what we do now), and a separate HTML file for each prefix group and an HTML file for each grouping.
- First pass at (hopefully!) useful groupings of stats.