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?
Why is this before the code block that starts in line 87?

The paper has it the other way round, and since `flag` is used in `_eikonal` it does matter.