Remove kew Contexts from shepherd

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

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

346aa898c3a4d4ab118430fd0e8ca11e6a4d2844 (2014-09-25 17:39:21 -0700)
Remove kew Contexts from shepherd
They have unclear semantics and moving to asynchronous promise execution has
been causing a number of issues.  This is clearer and safer code.

R=@x-ma
R=@nicks
R=@dpup