#18 aligners leak memory

[Aaron Darling]

During the course of alignment, the heap space used by mauveAligner and progressiveMauve accumulates to colossal proportions on long runs.

The ghastly memory leak is due to a failure of SlotAllocator::Free() to call object destructors. If an allocated object contains member variables that allocate heap space themselves, that heap space never gets freed because member variable destructors never get called.