Enable better control over Logging

I was frustrated with the inability to control my SPDY logging when `DEBUG=1`. This pull request makes 2 minor changes to improve the flexibility:

1. Respects any existing value for `SPDY_DEBUG_LOGGING` exported to the build environment
2. If a `sharedLogger` instance has been configured, it favors that logging interface over the fallback `NSLog` regardless of the value for `SPDY_DEBUG_LOGGING`