Add accessors for the inputs of a node, so that we can change the representation without breaking everything.

Hello @eford1, 

I'm mostly interested in feedback on names and how things are documented.

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

8e700245b229c3c0cb0302c4a12f9fbf78cff430 (2013-07-23 20:56:46 -0400)
Add accessors for the inputs of a node, so that we can change the representation without breaking everything.


R=@eford1