Added Jeroen on the contact page

# Why
As Jeroen is now an official member of the team, I've added him to the contact page. Since Frank has left us (for now :wink: ), I've removed him to avoid confusion.

# How does it look?
Well, sadly, @jakubgg never uploaded a gravatar of himself, and @jeroen-groeneweg didn't know yet, so it looks like this:
![screen shot 2014-09-22 at 23 32 09](https://cloud.githubusercontent.com/assets/106844/4364436/000f7594-42a0-11e4-809a-3b90c5383ccc.png)
But that's easily remedied if @jakubgg and @jeroen-groeneweg both upload their faces to gravater :wink: