spdycat: Add support for SPDY proxies

I've added support for SPDY proxy servers to the spdycat tool.
This means that with the modifications you can specify a specific SPDY proxy host & port via the -p,--proxy and -P,--proxy-port parameters.
This would change the usage (for proxy usage) of spdycat to (for example, using port 443 and requesting google.com): spdycat -p myproxy.mydoma.in http://www.google.com.

This is mainly a change to the spdycat.cc file but smaller changes were necessary within the spdylay_ssl.cc & spdylay_ssl.h files to support the proxy settings.

Please let me know if there are conceptional or source-code problems present.

Best