BUG: fix bug in confidence interval estimation of stats.boxcox.
TST: finish stats.boxcox_llf test for 2-D input.  Fix bug in it.

boxcox() doesn't accept 2-D input, leave it like that.
ENH: add tests and docstring for stats.boxcox_normmax().

Also refactor boxcox and boxcox_normmax so MLE can be used from the latter.

Changes the `brack` keyword in boxcox_normmax; it differed from that used in
boxcox(), keeping boxcox() results unchanged is preferred here.

This finishes the Statistics Review for all Box-Cox functions:
Closes gh-680
Closes gh-681
Closes gh-682
Closes gh-683
Closes gh-684