Log4j 2.x support

I wrote this rather similarly to the Logback implementation, but it's not as fleshed out. I was a bit confused by all the package splits and other OSGi-abuse, so I kept things minimal.

If there are any issues you need me to address first before you can accept this (e.g., copyright notices, CLA, whatever), let me know! I'm the only Log4j 2 developer who works on the OSGi side of things, so I'm definitely the guy to come to with questions and concerns regarding Log4j 2 code.