tumblr_backup: order the imports alphabetically

Part of PEP-8.
tumblr_backup: Handle HTTP errors during image download
tumblr_backup: generate and save a post with one call

The aim is to have a parameterless function that can be used as a work
unit in the thread pool.
tumblr_backup: untangle the globals when saving images

Globals are inherently not thread-safe, so we need to have separate
instances for posts that are handled in parallel.
tumblr_backup: save posts in background threads (fixes #24)

This makes sense because it is I/O-bound, especially if there are many
photo posts.

Results for bbolli.tumblr.com (1452 posts, 336 photos, average of 5 runs):

- before: 86.2 s
- after: 21.2 s
Merge branch 'gh-24'