tumblr_backup: add support for downloading audio and videos

Optional dependency to youtube_dl for external embedded videos.

On Debian: apt-get install youtube-dl

Also see http://rg3.github.io/youtube-dl/index.html
tumblr_backup: minimal cleanup of the previous commit
tumblr_backup: more minor cleanups for audio/video downloads
tumblr_backup: add two options to enable audio and video download
tumblr_backup: unify media download

All media are saved in media/. Use the same method to download photos,
audio and Tumblr-hosted video. Media are downloaded in 1MB-sized chunks.

NOTE: The old 'images' directory is left as-is, because old saved posts
may still point to images saved there. If you intend to do a new
complete backup, it makes sense to rename 'images' to 'media' first, so
that the images already downloaded can be reused.
tumblr_backup: documentation update
tumblr_backup: refactor file name generation
Merge branch 'gh-32' into api-v2

NOTE: The 'images' directory is renamed to 'media'.

The old 'images' directory is left as-is, because old saved posts
may still point to images saved there. If you intend to do a new
complete backup, it makes sense to rename 'images' to 'media' first, so
that the images already downloaded can be reused.