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?
Is this necessary, considering that `image` is guaranteed to be uint8, and `inpainted` is just zeros or the values of `image`? (I'm actually not sure if `_inpaint_fmm` could return values that are outside the input range---but I would hope not.)