Fix balancing issue chaimleib/intervaltree#17

I see what you were doing here now. I've added this diagram in the comments:

    ## my_heavy == child_heavy
    #    self     save
    #  save   -> 1   self
    # 1

    ## self[my_heavy].balance == 0:
    #    self     save         save
    #  save   -> 1   self  -> 1  self.rot()
    #  1  2         2