FEAT: Full 3d anisotropic implementation of marching cubes

Also includes surface area calculation algorithm from
generated mesh.
Convenient output to visualize with `mayavi.mlab`.
Efficient Cython implementation.
import `marching_cubes` and `mesh_surface_area`
add _marching_cubes.pyx to list of Cython modules to build
fix import of _marching_cubes
fix: bug in second `_append_tris` call in case 125
rename files to conform with naming conventions
fix import error from `_marching_cubes_cy`
Merge branch 'add_marching_cubes' of https://github.com/JDWarner/scikit-image
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
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
Remove unnecessary conversion step to numpy array
Remove duplicate import statement