Fix javadocs and add jvm doc gen to CI.

Review Request #2877 - Created Sept. 23, 2015 and submitted

John Sirois
2251, 2260
benjyw, gmalmquist, zundel
Previously we could have invalid javadoc ond/or scaladoc on master,
now CI checks there are no jvm doc errors.

An existing javadoc error is fixed so CI passes.  Also `ScaladocGen` is
fixed to only grab java_sources when it actually can.  No test is added
for the latter, but without the fix, CI goes red on `junit_tests` owning
scala code, but not exporting a `java_sources` attribute.  This issue
tracks a proper solution to `java_sources` attribute tests:

 build-support/bin/                            | 2 +-
 src/java/org/pantsbuild/tools/jar/  | 3 ++-
 src/python/pants/backend/jvm/tasks/ | 6 ++++--
 3 files changed, 7 insertions(+), 4 deletions(-)

Before the doc fix, CXX=g++ ./build-support/bin/ -fkmsrlpc ran
red, now it runs green. Additionally, and more to the point, a publish
dry run on src/{java,scala}:: now runs green.

CI went green here:

John Sirois
Eric Ayers
John Sirois
John Sirois
Review request changed

Status: Closed (submitted)