Add per seconds metrics to some Nginx stats

The following Nginx metrics:

* ``conn_accepted``
* ``conn_handled``
* ``req_handled``

are value that only increment. they show like: 

![graphite](https://f.cloud.github.com/assets/474302/45046/c56fe196-573a-11e2-822b-57677c8cc4b7.png)

it's not very usefull to see the variations of requests and connections in a period of time.

this add additional metrics. basically it divide the difference in metrics increments since last update by the numbers of seconds.