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 
Sorry for the drip of comments, I would also include the expected signature, `proc_img = process_func(raw_img)`, something like
```
process_func : function, optional
   callable with signature `proc_img = process_func(raw_img)`, which will be applied to the data from each frame
```