Fix overflow error in statistic and variance multiplier

In CLinearTimeMMD and CBTestMMD the statistic and variance multipliers
were computed naively which caused overflow problems. Got rid of that

Changed methods are

 - CLinearTimeMMD::compute_stat_est_multiplier()
 - CLinearTimeMMD::compute_var_est_multiplier()
 - CLinearTimeMMD::compute_gaussian_multiplier()
 - CBTestMMD::compute_stat_est_multiplier()
 - CBTestMMD::compute_var_est_multiplier()
Added libshogun example for B-test MMD
Merge pull request #2348 from lambday/feature/btest

Fixed overflow error in CLinearTimeMMD and CBTestMMD and added libshogun example for B-test