message: Implement service layer, stubs for #81

I've made some effort to wire the manager into the controllers, but
tests are currently failing.

pkg/message/message.go

@@ -0,0 +1,26 @@
+// Package message contains message handling logic.
+package message
+
+import (
+	"net/mail"
+	"time"
+
+	"github.com/jhillyerd/enmime"
+)
+
+// Metadata holds information about a message, but not the content.
+type Metadata struct {
+	Mailbox string
+	ID      string
+	From    *mail.Address
+	To      []*mail.Address
+	Date    time.Time
+	Subject string
+	Size    int64
+}
+
+// Message holds both the metadata and content of a message.
+type Message struct {
+	Metadata
+	Envelope *enmime.Envelope
+}