Add SLF4J log4j binding as a runtime dependency

This allows the Stitching plugin to run successfully directly
from your development environment. Without these JARs in the
runtime classpath, any usage of Bio-Formats fails because
Bio-Formats 4.4.8 does not use a new enough version of SLF4J
to correctly handle the case of no binding being present.

Starting with Bio-Formats 4.4.9 (not yet released), this will
no longer be an issue and we can remove these runtime deps.