Add option to modify command to be executed

The default command is "gnuplot -persist". On SmartOS, gnuplot
does not output to dumb terminal with -persist option.
cf. https://github.com/basho/basho_bench/issues/82
In such case, adding the option "-E gnuplot" will help.
Cosmetics: untabify and reorder
Merge pull request #106 from basho/ss-exec-command-option

Add option to specify exec command
Expand latency graphing of basho_bench results

Separate out the 99th and 99.9th percentiles into separate plots and
add a plot of maximum latency times. Also add titles to all graphs.
This change repsents a small cleanup and refactoring of work done by
Matthew Von-Maszewski.
Switch to using os:timestamp in the cs driver
Merge branch 'feature/expand-latency-graphing'
Merge branch 'refactor/use-os-timestamp-for-cs-driver'