Added a wrapper around ndimage's Gaussian filter

This version of the Gaussian filter

 * uses 'nearest' as the default boundary mode. This can be discussed,
   but I had the impression that for images this is the most relevant
   mode ('extending' boundaries)

 * has a `multichannel` keyword, so that each color channel can be
   filtered separately. For now no attempt is made at guessing whether
   the image has color channels or not.
DOC: corrected description of multichannel parameter
[BUG] iterable sigma and multichannel

+ docstring improvement
Gaussian filter function: changed the default value of multichannel

Now we try to guess automatically whether the image is grayscale or RGB
Merge pull request #712 from emmanuelle/gaussian_filter

Add a wrapper around `scipy.ndimage.gaussian_filter` with useful default behaviors.
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`
move `unpack_unique_verts` to Cython for speed
fix: symmetric ambiguous dual-plane cases require special treatment
fix error in triangulation of case 120
FIX: dimension ordering now correct (x, y, z)
FIX: volume now directly cast, docfixes, C-contiguous ordering
FIX: use memoryviews, docfix, and change tri_list to face_list
FIX: incorrect name for `unpack_unique_verts`, revert axis ordering
DOCFIX: clarify docstrings and output vertex dimension ordering