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?
Yes, line 137 for the initialisation and then in line 88 for incrementing the boundary into the unknown region. Here, BAND denotes the boundary of the region yet to be inpainted and the known region. This will keep shrinking/expanding with each iteration of ``_inpaint_point``.