Switch to Cask for dependency management

Convert from Carton, which is deprecated, to Cask and upgrade to a
version of Pallet that will work with Cask. This allows me to jettison a
bunch of legacy load-path related code since everything now comes in via
Cask.
Purge unused org-mac-message library
Setup auto-compile mode

This replaces the old auto-recomp mode that I had vendored. It should
ensure that elisp files are recompiled whenever they're saved or loaded.