Add handling for message reply via email

If the reply_email_domain configuration is set the messages plugin will
use unique Reply-To addresses for every email. They contain enough
information to reconstruct the message context and a hash to avoid
user forgery and spam.
A mail transfer agent must be configured to call the new rake task
foodsoft:parse_reply_email for incoming mails. The rake task requires
the receipt of the mail in the RECIPIENT variable and the raw message
via standard input. An example invocation would look like:
rake foodsoft:parse_reply_email RECIPIENT=f.1.1.HASH < test.eml

plugins/messages/db/migrate/20160226000000_add_email_to_message.rb

@@ -0,0 +1,6 @@
+class AddEmailToMessage < ActiveRecord::Migration
+  def change
+    add_column :messages, :salt, :string
+    add_column :messages, :received_email, :binary, :limit => 1.megabyte
+  end
+end