FIX: Do not rely on strides for contiguous arrays in signal.medfilt

When an array is contiguous in memory but has shape[dim] == 1, then
its strides[dim] is not really used, so it can be considered arbitrary
even if the array is contiguous. This is generally true for 0-sized arrays.

Even without a change in numpy, this code (at least with numpy <1.8.)
could segfault for one dimensional arrays with one element.
TST: Expand signal.medfilt test to fail for odd strided contiguous arrays

This is possible in numpy < 1.8. for one element arrays however in generally
one should never assume that ndarray.itemsize == ndarray.strides[-1].
Merge branch 'contig-strides' into master.

Reviewed as https://github.com/scipy/scipy/pull/378.