add _.complement

  - Should \_.complement take an optional context argument? Currently it does not; I followed underscore-contrib's lead here.

  - It's quite hard to come up with a straightforward example for the documentation which isn't better expressed with one of Underscore's existing complements (we can't use \_.filter, for example, given the existence of \_.reject). Interestingly, had Underscore included \_.complement from day one, we mightn't need \_.reject and friends.

  - Should redefining \_.reject, \_.omit, etc. in terms of \_.complement be part of this pull request?