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?
Before describing the algorithm, it might be helpful to define `KNOWN`, `UNKNOWN`, and `BAND`. Maybe something like:

> Each pixel is labeled as KNOWN, UNKNOWN, or BAND, where pixels with known intensities (outside of the `mask` region) are labeled KNOWN, unknown intensities  (pixels under the `mask`) are labeled UNKNOWN, and known intensities bordering the mask are labeled BAND.