Support HTTPS proxies

This adds support for HTTPS proxy servers, which means an HTTP proxy that is capable of proxying TLS traffic to the remote endpoint. This addresses issue https://github.com/twitter/CocoaSPDY/issues/33.

It adds support for only a single proxy server in the configuration. A PAC file downloaded from an auto-configuration URL could theoretically returns multiple proxy servers, which are expected to be attempted in succession until a successful connection is made. This is not supported here, but could be added in the future. Much of the code is written in a way to support this, all that remains is the SPDYSocket "_close" codepath (should try next proxy).

Manual testing has been done in the simulator, using the Mac's system proxy settings and the Charles software, including induced failures to connect. More testing needs to be done on real devices against real proxy servers.