Fast Marching Method for image inpainting

This has been ob my todo list for long.

Replaces: https://github.com/scikit-image/scikit-image/pull/663

Rebase was impossible, also it was necessary to reduce the number of commit messages anyway.

I created a commit message in behalf of @chintak. I hope this is fine with you Chintak?
I would add a sentence before this to describe inpainting. Maybe something like

> Inpainting fills-in image regions with unknown, or undesired, pixel intensities (defined by `mask`) with pixel values from nearby regions or from regions with a similar structure to the masked region. This can be useful for filling-in missing data or correcting blemishes, such as scratches, in an image.