tumblr_backup: add EXIF tags to pictures

This patch adds two functions:

* it includes the post tags as IPTC Keywords inside the post images
* it adds a command line option, "--exif", to add global keywords to all images

The goal is to keep the content creator(s) credits attached to the image.

It depends on pyexiv2: http://tilloy.net/dev/pyexiv2/overview.html
tumblr_backup: cleanup the previous commit
tumblr_backup: handle --exif only if pyexiv2 is installed
tumblr_backup: improve support for changing EXIF tags

Before, EXIF tags could only be added to an image. Using the special tag
'-' (dash, minus) removes all existing tags on the image.

The empty tag '' means to add only the post's tags to the image, but no
others.
tumblr_backup: add the --exif documentation
tumblr_backup: refactor the option handler callbacks
tumblr_backup: fix the image path when adding EXIF tags (fixes #14)
Merge branch 'gh-14'