Add an addFinally method to Deferreds, to do cleanup without inadvertantly disrupting uncaught error propagation.

Hello @nicks, 

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

473ee57c6b264549685a0e6fc46bf3683a793b89 (2014-08-19 12:07:35 -0400)
Add an addFinally method to Deferreds, to do cleanup without inadvertantly disrupting uncaught error propagation.

R=@nicks