failing test case for strided memmap
FIX: potential segfault when reconstructing non-contiguous memmap

The strided_from_memmap helper function failed to mmap enough data from the
backing file potentially causing invalid memory access.

The fix is to compute the bounding buffer size in the reduce_memmap function
and use that as shape argument for the memmap constructor and only then cast
the resulting array with provided shape and strides.
TST: check non-unit step slicing
Make the strided_from_memmap helper private
Wording
Update CHANGES.txt
Merge pull request #133 from ogrisel/memmap-slice-bug

[MRG] FIX: potential segfault on non-contiguous memmap instances