speed up _.each
code style fix
Merge pull request #1231 from liuyl/speedupeach

speed up _.each
Fixes #1232, descriptive code comment for flatten
Merge branch 'master' of https://github.com/jashkenas/underscore
Refactor group functions a bit.

* No need to call lookupIterator for _.identity.
* Hoist function creation to load time.
Merge pull request #1233 from braddunbar/group-refactor

Refactor group functions a bit.
changes null and guard check in _.first and _.last to reflect style used in _.initial and _.rest
Merge pull request #1234 from jessepollak/master

Changes null and guard check in _.initial to make style cohesive