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 -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
```