Prevent a TypeError when Content-Type is empty
Move code for handling unicode issues in embeds into link.py

media.py should have minimal (if any) knowledge of having to UTF-8
encode dicts before storing them on Things
Modify handle_timeout to support **kwargs
Make the rescrape API work with the caching media scraper

This is a temporary workaround until a new scraper_q request message
type is added that forces a rescrape regardless of the current cache
state.
Add a cache by URL to the media scraper

Pull media embeds and thumbnails from a cache when cache use was requested
and cached media are available.