Check JavaDoc with Checkstyle for all annotations
Move Checkstyle execution to a separate profile for Travis

It can be really annoying for contributors for always fixing the code
due to broken build by Checkstyle even before starting implementing a
feature. That's why the Checkstyle execution is moved to a separate
profile what will be used by Travis CI. Contributors are still
encouraged to run Checkstyle manually before pushing code, but Travis
will reject malformatted code in the PRs anyway.
Adding @return to all method JavaDoc

JDK8 JavaDoc needs @return on all non-void methods. Unfortunetaly
Checkstyle does not know about this
(http://sourceforge.net/p/checkstyle/bugs/543/), so we have to disable
the JavaDocMethod check for now.