Added CamelCase log level constants

This is simply a semantic change/addition because constants in go typically use camel case.

This is not backwards breaking. Going forward, people should use the new camel case variable names.

e.g.,

self.Logf(slogger.Debug, "this is a test")

You should not mix all caps log level constants with camel case constants.

I also added copyright (MongoDB) and license information (Apache 2.0). If this is not the correct license for this lib, let me know and I will update.