Add promises to graph.ready/graph.onReady, to reflect how we actually use it to do startup init

Hello @kylehg, @giannic, 

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

c4c3e248bf6737b73586ca26fcbc410b32fc8b1b (2014-07-25 16:06:51 -0400)
Add promises to graph.ready/graph.onReady, to reflect how we actually use it to do startup init

R=@kylehg
R=@giannic