Implement message delivery receipts

http://xmpp.org/extensions/xep-0184.html

With a /receipt command to request a receipt, and perhaps a config option to enable/disable the response to a request (and thus disabling its support in the disco#info too).

• Changesets:
  • Revision 5999b71c by mathieui on 2014-04-27T14:32:03Z:

Fix #2106 (implement message delivery receipts)

- two options request/ack_message_receipts
- two new theme parameters : CHAR_ACK_RECEIVED and COLOR_CHAR_ACK
- if a message has a receipt, the character is displayed between the
  timestamp and the nick, using the color