Move null check into lookupIterator.
Merge pull request #1354 from braddunbar/iterators

Move null check into lookupIterator.
Ensure first handles negative numbers gracefully.
Merge pull request #1355 from braddunbar/first

Ensure first handles negative numbers gracefully.
Merge pull request #1353 from lfac-pt/master

Implements the _. constant utility method
Adds the documentation for _.property