Add ioping-metrics.rb

This plugins runs `ioping` and parse the results. It supports ioping v0.6 and v0.8.

```
% sudo /opt/sensu/embedded/bin/ruby ./plugins/system/ioping-metrics.rb --help
Usage: ./plugins/system/ioping-metrics.rb (options)
    -C                               Use cached I/O
    -c COUNT                         Stop after count requests
    -d DEVICE|FILE|DIRECTORY         Destination device, file or directory (required)
    -D                               Use direct I/O
    -i INTERVAL                      Interval between requests in seconds
    -n NAME                          Name of the series (required)
    -s, --scheme SCHEME              Metric naming scheme, text to prepend to metric
% sudo /opt/sensu/embedded/bin/ruby ./plugins/system/ioping-metrics.rb -D -c 5 -i 0.1 -n home-maoe -d /home/maoe -s localhost
localhost.ioping.home-maoe.min  0.0002  1408946759
localhost.ioping.home-maoe.avg  0.0019  1408946759
localhost.ioping.home-maoe.max  0.0087  1408946759
localhost.ioping.home-maoe.mdev 0.0034  1408946759
```