New module: "date tag"

I just created a new module that automatically adds tags corresponding to the month and year (F and Y in php's date() syntax) that an image was captured on. It work on new images, modifications, and existing images via an admin task.

Source is available at https://github.com/mikeage/date_tag ; there's a pull request to add it to gallery3-contrib at https://github.com/gallery/gallery3-contrib/pull/139 if you want to follow along.

The next planned improvement is a user specified list of tags to add (e.g., "F, Y" for the default behavior above, or "M_y, D" if you want to tag all images with tags of the form Jan_12, May_08, Dec_99 and Monday, Wednesday, Saturday, etc).