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
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Add closing p tag

After discussion with sharell it turns out that some of the automated
 translation tools used for creating other ANTLR language targets look 
for closing tags (for one reason or another), so for this project 
the practice is to explicitly close all tags.
Merge pull request #719 from jeduden/master