[MRG] Label ranking average precision

The goal of this pull request is to add a first metric for multilabel ranking problem "label ranking average precision". The definition can be found in [Mining multilabel data](http://lkm.fri.uni-lj.si/xaigor/slo/pedagosko/dr-ui/tsoumakas09-dmkdh.pdf) at page 14 or in the documentation.

For the moment, I decided to add a new function instead of a new possible average value to the current `average_precision_score`.  But this could change. I am also open to suggestions for a shorter name.
Let me give this a shot:

```
Label ranking average precision (LRAP) is the average over each ground truth label assigned to each sample, of the ratio of true vs. total labels with lower score.
```

I would add a link to the paper in the docstring of this function too.  Maybe the formula too (but i'm -1 since the formula itself is very trivial, but the denom and num have to be explained in words)