Port allocator, Reflective invocation, Void to returning function adaptors

While the theme of this release was to improve the continuous integration infrastructure,
we also added a few minor features.

The Ports class implements block-based port allocation, useful when one does integration
testing and wants to test in parallel a service which binds ports, and/or needs a private
data directory.

The Extractors utility got a brand new invokeMethod() and getAccessibleMethod(),
allowing to break encapsulation with minimum fuss (similar to their xxxField()
counterparts). The parameter order of getAccessibleField() has been changed in order
to make it consistent with the other methods (should result in easy to fix compile error).

The Functions utility now provides the ret() and retnul(), which allow us to write
more expressive one-liners when we need to use a void function in a position expecting
a non-void expression.

From the non-functional side, now the tests are running on OS X, Windows and Linux under
both JDK8 and (currently failing) JDK9. We have also added a Coverity scan
on top of the existing Findbugs and Checkstyle checks.

See also bintray version, javadocs at Sonatype or on javadocs.io (in case OSSRH is unavailable)