fix shading of slf4j (#51)

The previous shading included slf4j-api and slf4j-simple in the shaded jar.
slf4j-simple was intended as a runtime dependency, and thus shouldn't have been included.
slf4j-api was intended as an optional non-shaded dependency, and thus shouldn't have been included.
Merge pull request #52 from davidmc24/fix-slf4j-shading

fix shading of slf4j (#51)