Future features will require MessageMetric to provide source.

Therefore, we implement the resulting type-safe insanity now, to not
disrupt the public APIs when the release is out.
Conflicts:
	splitlog-api/src/main/java/com/github/triceo/splitlog/api/MessageMetric.java
	splitlog-core/src/main/java/com/github/triceo/splitlog/DefaultMessageMetric.java
	splitlog-core/src/test/java/com/github/triceo/splitlog/DefaultMessageMetricTest.java
A bit more type safety

The previous commit fails on Java 6 for some reason.
Fix Javadocs