The Maven structure is:
-/
--dbutil (used by xmppserver)
--i18n (used by starter and xmppserver)
--starter (launcher and starter package, startup.jar)
--xmppserver (nearly everything in the src dir, openfire.jar)
--webadmin (admin plugin)
--plugins
---broadcast
---...

I've also added Maven plugins to mimic partial behavior of the Ant build (copying dependencies, bin/conf folder, ...)

This commit addresses issues OF-546.

I reconfigured the pom.xmls to work with the existing directory structure, (instead of default src/main/java), so that the current src folder stays untouched.

What it can do:
- Run tests successfully
- Generate JavaDoc
- Assemble most plugin jars (without JSP)
- Assemble admin plugin (without JSP)
- Generate a source zip/tar.gz for distribution (as the download page offers it currently)
- Build the startup.jar and openfire.jar, copy it in the right directory, so that Openfire can be run with `java -jar startup.jar`

What it can't do:
- Build executables for every platform (e.g. install4j, *.dmg, ...)
- Build some plugins (e.g. ofmeet) due to non-Maven dependencies.

Todos:
- Fix these issues
- Move source code to the correct paths, e.g. /src to xmppserver/src/main/java