XWIKI-12: Reorganize directory structure to match Maven2 best practices and cleanup build

* Started creating the core/ module and added the first version of a m2 build.
* Please note that that the m2 is NOT working at this time as more work needs to be done to sort out the dependencies.
* I'm committing this to show to everyone the path I'm taking, to gather comments and possibly help too... :-)

git-svn-id: https://svn.xwiki.org/svnroot/xwiki/xwiki/trunk/core@1387 f329d543-caf0-0310-9063-dda96c69346f