[WIP] #107 Add label annotation 

This will implement #107 feature.


Ok, I've added the 3rd case. It is a naive and simple implementation but I think it should do. 
But like I said I think that this "get xpath thing" should be in another library, one used by qa-tools and mink. Mink is much more advanced in this stuff and I we don't want duplicated code. What do you think about this crazy idea?
Also, I think we should add more tests to this stuff. There are so much possibilities that I feel like something could be missing.