Add 'message' field to personal contribution page (pcp) contributions

As well as allowing the donor to leave their name along with their donation to a personal contribution page, a nice feature would be to also allow the user a way of leaving a message to go along with it. These messages should be able to be optionally displayed in the honour roll (the scrolling box that list all the donations along with the name and amount). Admin (and probably the user too) would need the ability to delete/amend bogus messages to ensure the content of the site is appropriate.