allows date ranges when collecting images on the left hand side dialog

This small patch adds two new entries to the "collect images" dialog. "before" and "after", both take dates (or dates + time) like "2014:09:15" or "2014:09:02 10:47" as argument. It selects the images before or after (including the exact term - it uses <= and >=) the given timestamp.

When you combine both new options, you can easily search for ranges, like the entire last vacation. It goes together with all the other "collect images" options.

![darktable-date ranges](https://cloud.githubusercontent.com/assets/8949897/4435577/99ff618c-474b-11e4-9cb0-ac0265c8dfc4.png)