Fast3tree

Update the docstring for `fast3tree.query_nearest_distance`. When one of the points in the tree is located exactly at `center`, this function would return the distance to the *next* nearest point. This behavior is not directly obvious and the new docstring states this feature clearly.

Thanks to chto for noticing this.

- Several under-the-hood updates were made.
- License updated to GPLv3.
- First release after this project moved from bitbucket to GitHub.