Added TestLoggerRule

[SquirrelGrip]

Hi,
I really like the slf4j-test implementation of slf4j and it gets around a number of issues for testing logging in any environment.

But I am partial to JUnit Rules.
I prefer to setup up expectations in a rule and then have the rule take care of it for me. Plus I don't have to clear the logger (although I did see the Rule that was there to do that for me).

I have some more ideas around this rule feature:

• Strict/lazy checking (strict would fail if you had unexpected logs, lazy would fail if had expected logs and they were not logged, but wouldn't care if their were other logs).
• Regular expression matching of messages.
• Level At Least (atLeast(INFO), which would pass if the Level was Info, Warn, Error, etc)
• Ability to control the expected level from the Rule

Just some of them
Adrian