Allow set only when a key does not exist

Hello @nicks, @giannic, 

Please review the following commits I made in branch 'xiao-setnx'.

77f63949cf92314afd2d4155242c48f10d704fe5 (2014-03-23 00:01:02 -0700)
Allow set only when a key does not exist

R=@nicks
R=@giannic