Add ntpdate-metrics.rb plugin

This plugin runs `ntpdate -q <ntp servers>` then writes the offset and the delay to Graphite. It's similar to `ntpstats-metrics.rb` but works without running ntpd.

Example output is as follows:

```
% sudo /opt/sensu/embedded/bin/ruby ./plugins/system/ntpdate-metrics.rb --server 1.ubuntu.pool.ntp.org 
foo.example.com.ntpdate.offset        -0.000436       1404367740
foo.example.com.ntpdate.delay 0.02902 1404367740
```