Add OptionalAssert#isPresent, #isEmpty, #contains

Allow assertions on Optional.
Fix incorrect Javadoc

The doclint added in Java 8 is activated by default and it is very restrictive.
We can disable it (the maven-javadoc-plugin may do in the future, http://jira.codehaus.org/browse/MJAVADOC-387),
but for the moment as this is merely new code, we can "easily" respect it.

@see http://blog.joda.org/2014/02/turning-off-doclint-in-jdk-8-javadoc.html