Implement ImapFolderAgent.
Define expected payloads and reuse them.
Include "folder" in events.
Cover the "mark_as_read" option.
Check regexp validation.
Convert 'port' to integer.
Unbreak with Ruby <2.0 not having File::FNM_EXTGLOB.
Fix chef cookbooks to work with ubuntu 14.04

The primary issue was that libshadow-ruby1.8 is not
present in 14.04. The secondary one is that default
ruby in 14.04 is 1.9, so update-alternatives step
fails.

Fix:
* drop the requirement of libshadow-ruby1.8, since
we set 1.9 by default.
* see if 'ruby' points to 1.9 and update symlinks only
in case it doesn't
Elaborate on how to specify a case-insensitive pattern.
Mark duplicated mails as read even if event creation is skipped.
Document how this type of agent memorizes and deals with already seen mails.
Merge pull request #330 from racktear/ubuntu_14_04

Fix chef cookbooks to work with ubuntu 14.04
Merge pull request #328 from knu/imap_folder_agent

Implement ImapFolderAgent.