Implement Block-test

The B-test is a test for the two-sample problem for high-dimensions.

This is an implementation of:
- Zaremba, W., Gretton, A., Blaschko, M. B-tests: Low Variance Kernel
  Two-Sample Tests (NIPS)
Well, we can do that. But actually it doesn't matter. The ``kernel`` function is called only once for computing the ``MMD`` in the beginning; it is only stored for pretty prints.