Instantiate new entities with a given UUID.

Was asked about:
[here](http://slick.ninjacave.com/forum/viewtopic.php?f=28&t=6699&p=35346), [here](http://slick.ninjacave.com/forum/viewtopic.php?f=28&t=6165&p=33517), and [there](http://slick.ninjacave.com/forum/viewtopic.php?f=28&t=5764&p=32544)

It's very simple, just sets the entities UUID when instantiated (optionally).