Add EXIF support

Sorry for those many PRs but I like your tool and want to make it more useful for myself.

This change adds support for EXIF tags. For this, I first added a change that preserves the EXIF tags when the original images are processed. In a second step, I read the tags and add the tag information to each `media_ctx` dict as a new `exif` key. Unfortunately, the tags themselves are a little bit unwieldy and to avoid too much work in the template code, I added a `simple` dict to that `exif` key. The `simple` dict contains the most interesting tags: `focal`, `iso`, `fstop`, `exposure`.

This is what I can get with these simple changes: 
![EXIF data in the info box](http://i.imgur.com/rfXRSly.jpg)