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

db/migrate/20160226000000_add_email_to_message.foodsoft_messages_engine.rb

@@ -0,0 +1,7 @@
+# This migration comes from foodsoft_messages_engine (originally 20160226000000)
+class AddEmailToMessage < ActiveRecord::Migration
+  def change
+    add_column :messages, :salt, :string
+    add_column :messages, :received_email, :binary, :limit => 1.megabyte
+  end
+end

db/schema.rb

@@ -188,6 +188,8 @@ ActiveRecord::Schema.define(version: 20160217194036) do
     t.datetime "created_at"
     t.integer  "reply_to",       limit: 4
     t.integer  "group_id",       limit: 4
+    t.string   "salt"
+    t.binary   "received_email"
   end
 
   create_table "order_articles", force: :cascade do |t|