MockWebServer can now read headers sent by http2 clients

This knocks out some dents in HPACK (http2 header compression).

Along with unit tests, I ran the SampleServer example (with  `-Xbootclasspath/p:/path/to/npn-boot.jar`) and was able to run the [http2 ruby client](https://github.com/igrigorik/http-2) against it.

This doesn't perfect the HPACK impl or fully support it.  However, it seems far enough progress to start enabling http2 on MockWebServer.