Proposed changes to Badger::Log/Badger::Log::File

[klesteb]

I have made some changes that might be nice.

1) Decoupled the setting of the log level from the actual log output
2) Started using Badger::Timestamp to generate the timestamp
3) Added config variable for the change the callbacks for log output
4) Added config variable for changing the timestamp format
5) Added a method to change the callback for the log output
6) Added a "trace" level

I have not updated the documentation for this change, I have updated the tests for this change. This change
currently passes all tests. I will update the documentation if the change is accepted.