implement extra wait time for !join

i.e. it makes sure that there are at least X seconds of wait time left after a !join.

Feb01[18:03:40] +Zazie_Lavender | I think there should be an enforced 10 to 20 second wait after last !join
Slightly change message when last player leaves
Merge branch 'master' of git://github.com/xnrand/seerwolf