[MRG] Automatically group short tasks in batches

This is based on an online evaluation of the batch completion time to dynamically tune the size of the batch of tasks to dispatch at once to a worker.

TODO:

- [x] ensure tests pass on windows
- [x] update the change log
- [X] test with scikit-learn
This information should print only for fairly high verbosity level, as it is of little interest to Joe user. I would say a verbosity threshold of 10 or 15.