Add a function later to kew for asynchronous execution

Hello @nicks, @dpup, @x-ma, 

Please review the following commits I made in branch 'nathan-later'.

b71870bdf7b9542591bfeb0676a73146d108b6d1 (2014-09-25 14:40:50 -0700)
Add a function later to kew for asynchronous execution

R=@nicks
R=@dpup
R=@x-ma
`Function` or `function(Function, Array.<Object>)`