httpbakery: add HTTP checker
Merge pull request #15 from rogpeppe/011-http-checker

httpbakery: add HTTP checker
httpbakery: add assert argument to CheckRequest

Also change it to return the information declared.
Merge pull request #16 from rogpeppe/012-override-declare

httpbakery: add assert argument to CheckRequest
bakery: depend on checkers rather than the other way around