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?
In `` _init_fmm``, we are already initialising the boundary of the region to be unpainted as BAND. See line ``137``. Next, we evaluate the eikonal equation for the BAND pixels. The next code block pushes the "BAND" into the "UNKNOWN" or yet to be unpainted region.