clean implementation of _.omit; it can be even cleaner if we add _.not
add _.complement
define _.reject as the complement of _.filter
Merge branch 'complement' of https://github.com/davidchambers/underscore
Fixes #1503. Stops the bikeshedding.
use _.negate in definition of _.omit; see 112e082c0f9e859d36ee1252f602b3a87774c34a