imgurcache.py: rename comment retrieval functions
imgurcache.py: read cached comments and blend them with live updates.
search-comments.py: read a stream of comments, not comment blocks (big simplification).
imgurcache.py, search-comments.py: Use cached comments, and update them automatically.
imgurlib.py: move headers creation inside make_request()
imgurlib.py: add handle_status() for HTTP response statuses
imgurlib.py: check quota credits on each request
inspect-comment.py: print thread in reverse order, like it appears on the site.
comment-survey.py: get a sample of usernames and total number of comments
imgurcache.py: identify user cache by id, not username
imgurcache.py: fix bug keeping limit from being reached
search-comments.py: add not-match -v switch
comment-survey.py: add another level of randomness, catch quota exception
imgurlib.py: catch JSON parsing exception, print debug info
README.md: Update to reflect caching.