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").
I'm not sure how the class-based system would actually be better -- it's possible to recompose these functions just as much as it's possible to recompose classes.

auth_interval and auth_retry_times also I think assume that re-authentication is done more that once, but I don't think that's really necessary. I think just re-authenticating once automatically is good enough -- the general retry mechanism will handle any pathological cases of authentication failing multiple times.