Include uuid

[theduke]

I was a bit surprised to not find uuid in here yet.

It's pretty widely used and managed by alexcrichton, so I'd recommend including it.

https://github.com/rust-lang-nursery/uuid/

Regarding features to enable by default, I'd vote for features = ["v1", "v4", "serde"].

v4 is by far the most widely used, and only depends on rand, which does not add a new dependency.
v1 also only needs rand, so just enabling it seems reasonable.

If it's fine with you, @brson , I'll whip up a PR with examples.