Potential bug fix for joining.

When 2 players simultaneously attempt to join a game with already 3
confirmed players in it, the confirmed players should reject at least
one of them. Therefore, they need to check whether the number of players
which may end up in the game (as opposed to just those that are
currently in the game) doesn't exceed the maximum number of players.
Added vote requester to facilitate implementing voting mechanisms.
Rewrote join requester as a vote requester.
Moved string constants to separate Constants.
Merge pull request #24 from tgoossens/vote

Added vote requester to facilitate implementing voting mechanisms.
initial seesaw proposal
implemented seesaw locking model. Tested. Only flaw: simultaneous locking