Added recent messages function in campfire adapter.

This function returns recent message from current room.
Params:
  limit: number of messages returned
  callback: result
Example:
  robot.adapter.bot.Room(msg.message.room).recentMessages 2, (error, data) ->
    msg.send data

Campfire API (recent messages): https://github.com/basecamp/campfire-api/blob/master/sections/messages.md#get-recent-messages