[slf4j-dev] Prepared branch for slf4j 2.0

[Federico Fissore]

Hi Ceki

I prepared the branch for slf4j 2.0. I cannot make a PR that targets a 
non existing branch: can you make one?

Code is available at https://github.com/ffissore/slf4j/tree/2.0.0-SNAPSHOT

I've replaced occurrences of java 1.5 and 1.6 to 1.8 and I've included 
slf4j fluent

Let me know what you think

federico