Added DoWithUUID function

Hey.

I started working with the mc server code and this is my first pull request that adds #1579.
Please review before merging it.
Wrong name. The function should be called `DoWithPlayerByUUID`, the `Find` prefix means that we do a non-exact matching (and I really don't like that function and want to remove it anyway).

I'd even go as far as adding a `DoWithPlayerByName` alias for the current `DoWithPlayer` function and marking the `DoWithPlayer` function as obsolete, replaced with `DoWithPlayerByName`.