Use existing UUID generator and better tests

I don't have a convincing reason for the new implementation, so
using Math.uuid for exposing `Pouch.uuids`.

`Math.uuid` exposes three ways to generate uuids. The default
method used in Pouch.uuids is `Math.uuidFast`. If we don't have
any reason to keep all three, it would make sense to remove the
other two for the one we think will work out best.

Also, the Pouch.uuids is defined in pouch.js instead of utils.

Signed-off-by: Rohan Jain <crodjer@gmail.com>