MDL-46282 core: Add VERP API
MDL-47195 libraries: Add horde IMAP from Horde 5.1.2
MDL-47195 core: Add Horde to classloader
MDL-46284 core: Add e-mail fetcher from IMAP
MDL-3714 mod_forum: Add a VERP reply_handler for mod_forum
MDL-47197 core: Private files handler