Make session.ChannelMessages() accept MessageID as a string

[moxian]

Currently session.ChannelMessages() accepts before and after message IDs as strings. Everywhere else message IDs are strings, so to pass them to ChannelMessages() one has to call strconv.Atoi() or something similar, which is rather cumbersome.
Consider making ChannelMessages accept Message ID as a string instead.

One caveat is that this change will make it difficult to obtain the very last message (channel.LastMessageId), because ChannelMessages does not include first and last messages in the response. A workaround to that would be making utility functions to return an Message ID that immediately follows/precedes given one.

func NextMessageID(mID string) string {
  idInt, _ = strconv.Atoi(mID)
  return fmt.Sprintf("%d", idInt + 1)
}

or something along the lines.