Fix JDK8 incompatible comments

In JDK8 javadoc runs a lint pass over the documentation and fails if the docs
are not valid HTML. Also, see following article for a more in-depth 
discussion (http://blog.joda.org/2014/02/turning-off-doclint-in-jdk-8-javadoc.html)

Removed all occurrences of <br />,<p /> and replaced them with <br> and
<p>.
Require Maven-plugin-plugin version 3.3 

Reporting targets fail for dependants of the maven-plugin-plugin with
JDK 8, because the plugin generates invalid javadoc.
This has been fixed in version 3.3. 

Also see: https://jira.codehaus.org/browse/MPLUGIN-244