[GH-321] Better logging

[rlepinski]

What do these changes do?

This PR does a few things:

• Makes it so apps can override how Airship logs
• Improves filtering by using autoclosure blocks to prevent computing the message
• Bridge all objective-c logs to AirshipLogger so we no longer use NSLog.

How did you verify these changes?

Ran sample app, verified logs are coming through still.

[rlepinski]

Merging into our private repo, will be available next minor release.