#60 Add the ability to challenge a user directly

[Alf Mikula]

When viewing another's profile, and when viewing a list of other users, provide a link for issuing a direct challenge to another user. It should be possible to include a message with the challenge, as well as a location and a range. For example, "Any location within 5 miles of Cafe Nervosa". The challenged user should then be able to get a list of all locations that qualify for the challenge, and pick the one that works best. Alternatively, the challenged user can issue a counter-offer of her/his own location and range. If a location and time can be agreed upon, then a scheduled game will be set up between the two players.