tumblr_backup: use urlencode() to assemble the query arguments
tumblr_backup: make the tags link to other posts with the same tag
tumblr_backup: use meaningful image filenames

Tumblr image filenames are quite meaningless if one where to look at the
saved images locally in a file manager: there is no easy way to find
back the post related to a specific picture.

I propose to use a different pattern to save the images: account_postID.ext

This way, one could find back the post attached to the picture they were
looking at locally.
tumblr_backup: cleanup the previous commit
tumblr_backup: make the image filename format configurable (fixes #13)
tumblr_backup: fix save_image()'s docstring
Merge branch 'gh-13'

Conflicts:
	tumblr_backup.py