Update pick and omit to accept a predicate
Merge branch 'pick-pred' of https://github.com/jasonLaster/underscore into pick-omit
Update pick/omit to take a function callback.

* Clean up tests.
* Branch for iterator case.
Update documentation.
Revert call signature change.
Spacing.
Loops.
Merge pull request #1485 from braddunbar/pick-omit

_.pick and _.omit accept a predicate