Support ad-hoc keep-alive for HTTP/1.0 Clients

Only go for keep-alive if they submit a 'connection: keep-alive' header
in the request, keep behaviour the same otherwise.

The new RFC 7230 (http://tools.ietf.org/html/rfc7230#section-6.3)
states:

    If the received protocol is HTTP/1.0, the "keep-alive" connection
    option is present, the recipient is not a proxy, and the recipient
    wishes to honor the HTTP/1.0 "keep-alive" mechanism, the
    connection will persist after the current response;

Even though clients are discouraged from doing so in Appendix A.1.2
(http://tools.ietf.org/html/rfc7230#appendix-A.1.2)
Merge branch 'http-1-0-keepalive' of git://github.com/heroku/cowboy