Issue 53 cpu usage

@fandrei - My understanding of the code is that we should get one `NxlogProcessorUsage` CPU measuement every minute, but we seem to be getting closer to 18.
See http://kibana.cityindex.logsearch.io/#/dashboard/elasticsearch/LogSearchShipper%20-%20QAT%20ProcessorUsage
![image](https://cloud.githubusercontent.com/assets/227505/4991689/9e3d30a2-698e-11e4-905c-9cc6b570a50d.png)

The same seems to be true for `NxlogProcessorUsageNomalized`, `ProcessorUsage` and `ProcessorUsageNomalized` measurements.

Could you figure out why this is the case, and only report 1 measurement every 60 seconds. (ie, 4 measurements in total, 1x ``NxlogProcessorUsage`, 1 x `NxlogProcessorUsageNomalized`, 1 x `ProcessorUsage` and 1 x `ProcessorUsageNomalized`)