DOC: reformat docstrings, code to standards

Reformat docstrings and expand docstrings for numpy docstring standard.

Change code spacing to PEP8 (remove blank lines, mainly)
RF: refactor imports to standard order

Put imports in standard order; use relative imports where possible.
RF: remove binary_threshold

binary threshold is a one-liner in numpy; use that line instead.
DOC: extend applymask docstring

Make applymask docstring more explicit
DOC: fixes to median_otsu docstring

Fixes suggested by Stefan VdW
Merge pull request #237 from matthew-brett/refactor-median-otsu

WIP: cleanup docs / small refactor for median otsu