Add south migrations the "images/focal_point" media extension to automatically add the "focal_x/y" fields to FileNode

[boris-chervenkov]

When the project uses south, it's much easier to let south handle adding focal_point fields to the FileNode.

[rosscdh]

Would be nice to have this pull merged?

[samluescher]

Sorry guys, completely overlooked this! It's a good idea though, would you mind adding a note to the docs to include with this PR?