JENKINS-15539 serve jacoco.exec over HTTP

See https://issues.jenkins-ci.org/browse/JENKINS-15539 for the original issue. This change exposes the collected jacoco.exec file over HTTP. It is smart enough to merge multiple exec files if they are available, so that clients do not have to deal with this complexity.

This change contains a bit of refactoring so that the knowledge about how the files are placed in `$JENKINS_HOME` is concentrated in one class. I believe this also simplifies the code (by reducing the amount of `File`s and `FilePath`s that are flying around), but I also realieze that this is subjective, so I'll leave it up to you to decide if you merge this or not.