add shepherd primitive for singleton-scoping

Hello @dpup, @chaosgame, 

Please review the following commits I made in branch 'nick-singleton'.

e4c17705e03f300100d54c42a21720dc943d490a (2014-07-25 17:28:23 -0400)
add shepherd primitive for singleton-scoping

R=@dpup
R=@chaosgame
it's internal, so we can change it. what should it be called?