Fixed whisper messages

* Whisper protocol wasn't properly suppling envelope slices
* Message history wasn't properly propagated
* Added 'Messages' method, filtering any current envelope with the
  supplied filter.

whisper/message.go

@@ -67,7 +67,11 @@ func (self *Message) Seal(pow time.Duration, opts Opts) (*Envelope, error) {
 		}
 	}
 
-	envelope := NewEnvelope(DefaultTtl, opts.Topics, self)
+	if opts.Ttl == 0 {
+		opts.Ttl = DefaultTtl
+	}
+
+	envelope := NewEnvelope(opts.Ttl, opts.Topics, self)
 	envelope.Seal(pow)
 
 	return envelope, nil