mail_export: add support for custom domains
tumblr_backup: improve logging
tumblr_backup: handle link posts without a link text (fixes #6)

The default text is the URL itself.
tumblr_backup: handle image download errors gracefully (fixes #5)

In case of image download errors, the original image URL is kept in the
post.
tumble: switch to an OO design
Merge branch 'master' into samedir

Conflicts:
	tumblr_backup.py