Solved the Great Response Crisis of Week 8

By coercing lifetimes instead of types and moving from_http to its own trait, we have
managed to both allow new Response representations and remove all the lifetimes from
everywhere.

Such is the power of Sundays.
This was my original approach (you can see the remains on my great-response-crisis branch of my fork, if you're curious) but that doesn't work for the reasons @zzmp discussed with you on #iron last night.