Performance improvement. Stream the output.

There's no need to hold the entire output in memory as a single string. This change will produce annotated output line by line.
code duplicate reduction
Introduced JacocoReportDir to encapsulate the directory structure on $JENKINS_HOME
[FIXED JENKINS-15539]

/jacoco.exec now serves the (possibly combined) jacoco.exec file.
Merge pull request #17 from kohsuke/master

JENKINS-15539 serve jacoco.exec over HTTP