Configurable header compression

[dmcgowan]

Currently the spdy framer uses the best compression setting for zlib, which is also the slowest. The compression should be configurable on connection creation to allow for less overhead during spdy stream creation. In libchan profiling switching from BestCompression to BestSpeed reduced the time overhead of compression from 24% to 4% for the same test. Since spdystream is designed to be used over any type of net.Conn, this should be configurable to match the net.Conn speed capabilities.

Changing this will involve vendoring and altering the go.net spdy package. However the spdy package appears to be frozen so I don't think that is a problem.