HTTP 2.0

I've just put a small patch in branch chris/http2 to enable libcurl to
use HTTP 2.0 if it is the latest version.

libcurl also needs building against nghttp2 for this to do anything
(there is an equivalent patch in the toolchains chris/http2 branch)

As it is still in draft I've put an option in to enable it, rather
than it being enabled by default. Certainly Google's server - if not
connecting over https - returns an error. Over https it works fine
(and appears to have negotiated http2 to some extent), so I think it's
probably just not up-to-date with the latest spec.

I'll temporarily add myself to the curl mailing list and ask about
that.

Anyway, it's there for testing, merging, criticising, whatever.

Chris