Do not accept merge request if contains `WIP` label

I use the WIP MR at the beginning of the work

WIP MR should not be merged.
But I (or other) submit the accept button by mistake often.

So I hide accept button if MR title include "WIP"

![2014-04-10 1 33 47](https://cloud.githubusercontent.com/assets/608755/2658296/9bc371f6-c006-11e3-88c6-b6301012150e.png)