Update SPDY error handling with HTTP/2.0 changes.

* Copies the stream error handling and error codes from HTTP/2.0 draft
spec.
* In doing so, provides a single list of error codes.

TODO: Remove references to old error codes.
TODO: Update session error handling section too when confusion in that
section is fixed: http://lists.w3.org/Archives/Public/ietf-http-wg/2013AprJun/0719.html
Update RST_STREAM with HTTP/2.0 draft text.

This eliminates the error codes there and references the error code
section.
Update the GOAWAY frame and SessionErrorHandler sections.

Purely editorial changes. Note that the GOAWAY framing change was in an
earlier commit.
Merge pull request #13 from willchan/ErrorCodes

Error codes