Save depth images (cleaned up)

[peteflorence]

This is a cleaned up version of https://github.com/RobotLocomotion/LabelFusion/pull/10 as discussed there

Tested and working.

Thank you @5yler for implementing the saving of the depth images!

[peteflorence]

Note that python's argparse does not support booleans so the script logic had to have a few lines to do that manually https://stackoverflow.com/questions/15008758/parsing-boolean-values-with-argparse