ENH: Add convenient greyscale conversion.

This adopts `as_grey`, taking both the British-style naming and the smart calibration (assuming contemporary CRT phosphors) from similar functionality scikit-image.

This includes new tests and an overhaul of all the docstrings.

Closes #48 
To be pendantic, the test is `callable` so an object with `__call__` defined with pass, but I would not call it a function per-say.