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 don't really understand this comment. I would suggest it either be clarified or removed. Maybe something like

> # Flag outside pixels as 2, band pixels as 1, and unknown pixels as 0.