Parse proxy-style (absolute-url) requests

According to http://tools.ietf.org/html/rfc2616#section-5.1.2 it's valid to use absolute urls in the start line even for non-proxy requests, and servers MUST support it. Tornado needs to parse the url if it doesn't start with a slash (this will need to be messaged for people who are currently writing http proxies with a `.*` routing rule)