Test refactor for simplicity + adding test for silent dependency behavior

Hello eford, sho, 

Please review the following commits I made in branch 'jeremy-silentDepsHashing'.

5459e9241a62f3ba2e466505cf9c21e736f30924 (2013-01-04 15:16:37 -0800)
Adding testing that silent dependencies are taken into account for hashing of nodes
better comments

R=eford
R=sho