Add "exact" recommendation to permutation test

[rasbt]

As described in #721

The [permutation_test] function could raise a warning if the user wants to run a number of simulations that is close (> 50% ?) to the total number of possible combinations, so that they can switch to the exact method in that case.

[pkaf]

This would be a rare case no? Even for a m + n = 100 and m = 50, the total number of combinations would be massive.
However, I see in a case of m<<100 there is a room for an improvement. Am I understanding this problem correctly?

I am happy to raise ValueError(?) there if you wish.