Implements the _. constant utility method

Higher-order identity function.

As proposed in #1345
Added an alias and refactored _.constant
Adds _.constant to the documentation page
Removes the _.const alias
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
Merge pull request #1358 from lfac-pt/master

Adds the documentation for _.property