Hide more flags from SPDY APIs.

Flags are not portable between SPDY/3 and HTTP/2.0,
so hide them in the reader & writer code.

Also rename SpdyReader to FrameReader and SpdyWriter to
FrameWriter. Other classes will be renamed as they become
independent of the underlying protocol.
Merge pull request #283 from square/jwilson/hide_more_flags

Hide more flags from SPDY APIs.