Add --frames arg to okcurl, which sends HTTP/2 frame metadata to STDERR.


Also reworks SpdyServer to also work with HTTP/2.

```bash
$ okhttp/okcurl/target/okcurl-2.0.0-SNAPSHOT-jar-with-dependencies.jar --frames -k  https://localhost:8888/ >/dev/null
>> CONNECTION 505249202a20485454502f322e300d0a0d0a534d0d0a0d0a
>> 0x00000000     5 SETTINGS      
<< 0x00000000     0 SETTINGS      ACK
>> 0x00000003    60 HEADERS       END_STREAM|END_HEADERS
<< 0x00000003    50 HEADERS       END_HEADERS
<< 0x00000003  1468 DATA          
<< 0x00000003     0 DATA          END_STREAM
>> 0x00000000     8 GOAWAY  
```