Fix missing Processors namespace

Into Presentation::processors method, some processors are used
(HrToSections, ImpressJs, RevealJs). Those processors are defined
in a module Processor. By doing so, it's impossible in that scope to
resolve their name without providing their namespace which is
Processors.

Issue plexus/slippery#1
Merge pull request #2 from toch/bug/fix-missing-namespace-Processors

Fix missing Processors namespace