verify mavenn request (look at x-mavenn-sig)
set stream active when tuples
run HNBot.post_activity every 15 min
only post from mavenn initiated streams
delete streams if 410
set JSON body and application/json
handle when avatar is bad or missing
Crawler accepts a prefix url which remains unchanged
Base Url is used to crawl.
Moved link extraction to Link as class methods
Link.extract_* can can be reused in List crawler.
Prune unused code in Comment, Posting and Stream