Compute resources histograms

As with the cluster tool, use -D, -L or standard input to specify the
summary list.

Example use:
find /somedir -name "*.summary" | xargs grep -l -e "^command: SOMECOMMAND" | ./histograms output-dir

Use -s to split summaries according to categories (commands if
categories are not defined).

Generate *.stats per category, with max, min, avg, std dev, and
quantiles. (format that makeflow reads for the resources loop).

Generate histograms (png + html) to inspect outliers).

Requires gnuplot > 4.6
Move from apps to resource_monitor
Update resource_monitor Makefile