NEXUS-5488: Adding log4j-over-slf4j to core

Also, made nexus-webapp explicitly reference
the three SLF4j libs (api, jcl-over and log4j-over)
to be clear they are being includded into resulting WAR
Merge pull request #736 from sonatype/nexus-5488-log4j-over-slf4j-to-core

NEXUS-5488: Adding log4j-over-slf4j to core