add conditional request support to polling
add conditional request support to polling

as described in https://developer.github.com/v3/#conditional-requests

by using etags, we allow github to return '304 not modified' responses,
which don't count against our rate limit. This should help fix the rate
limit issue we discovered tonight.

pangaunn/hackmap,pangaunn/hackmap,bkkhack/hackmap,bkkhack/hackmap