initial implementation of jensen-shannon (in pure-python mode only)

Not positive that the results are correct; needs further checking /
tests to be written.

Modified interface for estimator functions some; Cython code doesn't
support the changes yet.
fix a bunch of math bugs in the jensen-shannon estimator
add a test that 1d JS estimate is right
tweaks
turn off wraparound in _np_divs_cy
_np_divs_cy: Ks is always ints now, drop fusedtypes
working cython JS estimator :)