Add cube selem.
Add ball selem. I verified in 2D that this usage of np.mgrid yields the exact same results as np.meshgrid (np.meshgrid is not available for 3D).
Fix typo in docstring of disk().
Adding octahedron selem (3D equivalent of diamond).
Added test for ball and octahedton selems.

Note that only the center slice (in each dimension is tested)
since there is no reference data available for these selems.
I did visualize the 3D shapes to see whether they look right.
Tony's comment on output format
Update release instructions for new robots.txt file
Inline helper functions of label
Add option to return number of labels
Add test case for number of labels
Rename development guidelines, so they are shown when opening a new PR
FIX use specific structuring element in Canny
Merge pull request #498 from JDWarner/fix_canny_strel

Use specific structuring element in Canny function
Merge pull request #497 from ahojnnes/guidelines

Rename development guidelines, so they are shown when opening a new PR
Merge pull request #484 from almarklein/3d-selem

Adding predefined structuring elements for 3D morphology.
fix to preserve input image shape upon executing `random_walker`
PEP8 fixes; remove unneeded pyamg import; add `data` shape check
Use `force_copy` argument of `img_as_float`

Also three PEP8 fixes
Make return type of skeletonize function boolean
Reformat code to comply with PEP8