Add thenScoped() and failScoped() as an efficient convenience for using instance methods as callbacks

Hello @nicks, @majelbstoat, 

Please review the following commits I made in branch 'dpup-thenScoped'.

539081519ebb7cfdbdbef04c9db31e38780332ee (2014-02-06 10:53:57 -0800)
Add thenScoped() and failScoped() as an efficient convenience for using instance methods as callbacks

R=@nicks
R=@majelbstoat
VISIBLE_CHANGES=Add thenScoped() and failScoped() as an efficient convenience for using instance methods as callbacks