UUIDs, new player data format, Java 6

* In order to prepare for name changes, CraftZ now uses UUIDs instead of
player's names wherever it's possible.
* A new player data format is introduced, this is [1] now mapped to
UUIDs (as described above) and [2] should result in smaller files. A
converter is included.
* The compiler's compliance level was downgraded to Java 6 in order to
be compatible with all servers.