remove guava dependency

Hi Robert,

I searched for code to dynamically choose slf4j's log level and came to your project which looks really nice.
I am building a small CDI interceptor lib at the moment which could be built on that basis.

Personally I do like Google guava a lot, but do not want to require the users of that generic library to include guava in their artefacts. Because of that I replaced `ImmutableSet` by `EnumSet` in combination with `Collections.unmodifiableSet` in my fork.
The code can be read almost as good as before and behaves strictly the same.

Maybe other users would prefer the reduced dependencies, too. :-)

PS: This is my first pull request, so if there is anything missing or wrong please do not hesitate to tell me about it.