Effect-based HTTP client with re-auth and retry support

This should be good for review again.

Note that Effect 0.1a5 now has a generic "retry" function that knows how to retry any effect, which means this code doesn't actually need to do anything to support retries (other than use Effects).

_some_ of the code in otter's retry.py may be useful in that context, but a lot will probably need to be refactored to be useful. (the existing retry.py doesn't support "should retry" checks that return Deferreds, so it's not really useful for the important use case of "check to see if the previous request actually had an effect").
Out of curiosity, what is wrong with `req_headers = headers or {}`?