ENH: Added binary_mask as input parameter for grid3d

This is the a mechanism that I came up with for masking
bad regions of images that I think @tacaswell will agree with.
The other approach would require masking before process_to_q.  If
we go down that route, then process_to_q will need to take in the
image stack again.
MNT: binary mask application to q is working, thanks @tacaswell
MNT: More memory efficient binary_mask creation
MNT: Binary mask needed a ravel().

almost had it @tacaswell!  Not bad for off the top of your head
DOC: Put todo's in re: masking
MNT: Fixes based on @tacaswell's comments.
Merge pull request #139 from ericdill/grid_mask

ENH: Added binary_mask as input parameter for grid3d