STATS job cleanup and fixes

@JavierJia @anbangx @Elmira88 @Nan-Zhang 

STATS is a pattern that runs a MapReduce job and creates a text file, `stats.txt` with a bunch of hadoop counters in it.  We also draw histograms of several features.

Current stats calculated are histograms, totals, and maximums for:
* # of nodes
* degree
* kmerLength
* coverage
* # of startReads
* # of endReads
* # of readids across all my edges
* # of self-edges (this one may have a bug right now?)
* # of path nodes
* # of tips, broken down by tips-FORWARD, -REVERSE, -BOTH and -ONE

I think it would be nice to have the N50 and family here as well, merging in @Nan-Zhang 's code.