Feature graphiteDataWriter write interval

Hi Gatling team,

I talked to Stephane earlier about having a configurable push interval for the GraphiteDataWriter. He asked me if I could contribute this feature myself. Although I am not a developer, let alone a scala developer, I asked a colleague of mine to have a look at it.  It turned out he had no time available after all so I had a look at it myself and managed to add the feature myself :-) 
This is my first ever pull request to an opensource project so please have some patience with me if not everything is perfectly done. I have modified the existing documentation slightly, not sure to what extend you want to document a simple feature like this. I did not add any unit test, simply because I don't know how to do this (yet). If you could point me into the right direction I can add tests if necessary.
I have tested the change in my Gatling-Graphite setup and it seems to do what is was designed for :-)
If you have any questions/remarks please let me know

Cheers 

Daniel