Scrub StatusLine from the public API.

The status line is an unnecessary grouping of three otherwise-independent
fields: the protocol, code and message.

Combining them in the API doesn't provide much value. In SPDY and HTTP/2
these are broken apart anyway.
Merge pull request #730 from square/jwilson_0419_status_line_is_not_a_thing

Scrub StatusLine from the public API.