http://jira.xwiki.org/browse/XDOCKER-11

[Misc] Fix typo in docker-entrypoint.sh that prevented using a custom mysql user name + Removed unnecessary env variable at runtime

[Misc] Go back to the previous setup which was better and I've found a way to make DockerHub happy with it.

[Misc] Since we may want to generate different images of xwiki in the future (with different dbs, different servlet containers, etc), move all current data into a xwiki-mysql-tomcat directory.

[Misc] Improved README + give a name to the xwiki-data volume + add LICENSE + improved .gitignore file