Let's also have a log4j2 implementation that redirects everything to
JUL, without the whole log4j-core.
Added log4j-provider.properties to describe our implementation. Also set
up pom so that the artifact can be easily used either as implementation
or appender.
Add a format to JULLogger with which it can pass the Marker to the
Logger from JUL.
Added missing license headers.
Marked the log4j-core dependency as optional.
Updated the readme.