Replace zip with itertools.izip under Python 2

This reduces the amount of memory required for complicated calls to
Variable.__getitem__ under array indexers, which iterates over all
indexers with "zip".

netCDF4-python's handling of array indexers is still very complicated/slow
when there are multiple array indexers involved, but this at least improves
the situation.
Merge pull request #234 from shoyer/reduce-getitem-memory

Replace zip with itertools.izip under Python 2