inLimbo -> isOpening

inLimbo == !isOpen && !isClosed, which is the same as the
state 'new' or 'opening', but for the same reasons of
removing the CloseError, we never return the LevelUP object
in the state 'new', therefore inLimbo really means isOpening
Merge pull request #75 from rvagg/feature/limbo-equals-opening

inLimbo -> isOpening