tumblr_backup: fix error while d/l protocol agnostic inline images

Fixes #30.

Got this error while doing python tumblr_backup.py -t me nikkiediamond
(NSFW!):

Traceback (most recent call last):
 File "/usr/lib/python2.7/threading.py", line 552, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.7/threading.py", line 505, in run
    self.__target(*self.__args, **self.__kwargs)
  File "tumblr_backup.py", line 799, in handler
    work()
  File "tumblr_backup.py", line 559, in save_content
    append_try('regular-body')
  File "tumblr_backup.py", line 549, in append_try
    self.get_inline_image, elt
  File "/usr/lib/python2.7/re.py", line 151, in sub
    return _compile(pattern, flags).sub(repl, string, count)
  File "tumblr_backup.py", line 658, in get_inline_image
    saved_name = self.download_image(image_url, image_filename)
  File "tumblr_backup.py", line 675, in download_image
    image_response = urllib2.urlopen(image_url, timeout=HTTP_TIMEOUT)
  File "/usr/lib/python2.7/urllib2.py", line 127, in urlopen
    return _opener.open(url, data, timeout)
  File "/usr/lib/python2.7/urllib2.py", line 393, in open
    protocol = req.get_type()
  File "/usr/lib/python2.7/urllib2.py", line 255, in get_type
    raise ValueError, "unknown url type: %s" % self.__original
ValueError: unknown url type: //i.imgflip.com/8kv0m.gif [NSFW!]
tumblr_backup: mark threadpool tasks as done if they throw an exception
tumblr_backup: DRY the format of the archive file name
Docs: type fix
tumblr_backup: handle more errors while downloading images

Fixes #45.
tumblr_backup: use double quotes for strings containing english

That's just my convention, but I like it :-)

Single-quoted strings are technical and not translateable.
tumblr_backup: fix a syntax error in Python 2.7

Fixes #46