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)
Ehm yes or just ``stderr = std(hh)/sqrt(n2)`` to be consistent. Thx.