Implemented ImageZigzagPersistence * Changed ZigzagPersistence to support a visitor, and implemented ImageZigzagPersistence * examples/rips/rips-zigzag now computes using ImageZigzagPersistence * PersistenceDiagram no longer records zero persistence pairs * Added utilities/memory.h with report_memory() function

add_subdirectory			(alphashapes)
#add_subdirectory			(ar-vineyard)
add_subdirectory			(cech-complex)
add_subdirectory			(fitness)
#add_subdirectory			(grid)
add_subdirectory			(triangle)
add_subdirectory			(poincare)
add_subdirectory			(rips)