Add NewUUID() helper

I intend to use this for constructing random URL paths so that we don't
encounter any cache poisoning between tests, e.g.

GET /de3ae9d3-7be5-42ac-b706-194b7a4b2c0a
GET /1cd37e36-d9c9-4ba1-9561-ae85f8fc54d4
Merge pull request #3 from alphagov/uuid_helper

Add NewUUID() helper