3.0 add warning when headers has been sent

Add a warning if the headers have already been sent.
![capture d ecran 2014-09-13 18 54 47](https://cloud.githubusercontent.com/assets/4977112/4261271/cfeb694e-3b66-11e4-9aef-6ed09078839d.png)

Let me know if you want to modify the design and/or if it wasn't a proper/best way of implementing it.