Bug 834757 - Remove [noscript] from nsIMimeConverter, part 1: move the encoders to a C++-only interface. r=irving, sr=Neil.

--HG--
extra : rebase_source : dd1c2ad440fb74e4b5503e2a210a8e2077d0bb39
Bug 834757 - Remove [noscript] from nsIMimeConverter, part 2: remove the last method. r=irving, sr=Neil.

--HG--
extra : rebase_source : 615870f2009551f1ad698b98bda1a4a0c5c8c727