tumblr_backup: URLErrors are also IOErrors
tumblr_backup: fix thread termination issues

There's no way around an event that can be set to tell the threads to
stop. It may take a while, though, because each thread will finish
saving its current post with all images before quitting.

That's why there's this cute cleanup animation.
tumblr_backup: document option -k
tumblr_backup: explain about duplicate inline image names
Merge branch 'master' into api-v2

Conflicts:
	tumblr_backup.py
tumblr_backup: show the note count and post source (fixes #29)

The v2 API also returns the note count and the post's source, so show
them after the tags.

This commit introduces the first functional difference between this and
the master branch.