Somehow missed a search 'n' replace
Fixes issue #1, an off-by-one error in img slicing

* Column slicing had an off-by-one error that meant some 1-px wide
columns weren't checked for foreground/background edges. This is fixed.
* The slicing of contiguous rows was more complex than it needed to be.
Finished renaming bg_sample_size config var in tests
Added test to ensure fix for issue #1

Ensures column blocks overlap by one pixel so that
`np.argmax(img) == 0` being interpreted as "no edge present" is guarded against.

This closes #1.