ENH: stats: Add Mood's median test.

This test is described in many statistics text books.  Google for it and you'll find many descriptions and tutorials about it.  It's about time we had it in scipy.  Since we already have the contingency table code, the implementation is pretty simple.
If you use "Sample" (as in the docstring) instead of "Argument", then there is no ambiguity in how to interpret the index (0 versus 1 based)