Remove the redundant acknowledgement in README.md
tumblr_backup: fix pagination links with option -r (fixes #35)

`self.archives` is already sorted according to `options.reverse_month`, so
the arguments to `next_month()` can be constants.
tumblr_backup: add download of inline images

This patch also saves images that are linked from other post types than
photo.
tumblr_backup: refactor image downloading
tumblr_backup: implement option -k for inline images
tumblr_backup: document inline image download
tumblr_backup: reverse the skip_images option internally

No UI change! The new option is called "save_images".
Merge branch 'gh-33'