Add plot scripts which use gnuplot

This pull request adds scripts to plot throuput and latencies from
basho_bench's test result.
Gnuplot is easy to install/use in several environments,
so it is a good complement to R.

Simple usage (after some test run):
```
$ priv/gp_latencies.sh
$ priv/gp_throughput.sh
```

Both scripts accepts `-h` option to print full usage.