Upgrade Doxia's checkstyle to version 5.x, and make it immediately useable within IDEs

The src/main/resources/config/doxia_checkstyle.xml document is tailored for checkstyle version 4.x.
Some incompatible alterations were done in the upgrade from checkstyle version 4 to 5
(refer to http://checkstyle.sourceforge.net/releasenotes.html for changes), implying that anyone
running IDEs or by other means running checkstyle version 5+ will crash and burn.

Moreover, the checkstyle document really ought to be directly usable within an IDE - or we should provide
some documentation on what needs to be done to make it run within IDEs. (More specifically, which properties
should be defined within the IDE and which values they should have). In particular, the RegexpHeader

<module name="RegexpHeader">
        <property name="headerFile" value="${checkstyle.header.file}"/>
</module>

... requires a property value, referring to a file not present within the doxia
VCS tree. We should really make maven download the jar containing the header file template,
and expand it. This would facilitate its proper use within IDEs.

I'll attach a raw, initial attempt at a checkstyle 5.x-compliant version of the doxia_checkstyle.xml