Factory based instantiation + linux fixes

Changes:

• Fix crash related to @autoreleasepool on linux.
• Fix log entries source location filepaths being fully logged, now only the relevant filepath information is logged.
• Add a configuration file from which you can control if timestamps are logged to your log file or not.
• Switch to a factory based instantiation system.

The bootstrapping now uses a factory and a config file, it looks like this:

try! StackdriverLogHandlerFactory.prepare(with: .init(logFilePath: "/var/log/my-app.log", 
                                                      defaultLogLevel: .debug,
                                                      logTimestamps: true))
LoggingSystem.bootstrap { label in
    return StackdriverLogHandlerFactory.make()
}