[MRG] Add benchmarking script for multilabel metrics

These are not very important metrics in the context of scikit-learn. Yet whenever metric implementations gets changed, people seem to be interested in how it affects execution time. This makes such reports easy to calculate.

This benchmarks metrics for different multilabel target formats, also giving us an idea of their relative performance. Benchmarks are otherwise parametrised by (number of samples, classes, average density of positive labels), one of which may be plotted against time.