[Validator] New validator for UUIDs

This PR adds a new validator for [RFC 4122](http://tools.ietf.org/html/rfc4122)-compliant [universally unique identifiers (UUIDs)](https://en.wikipedia.org/wiki/Universally_unique_identifier).  This implementation validates both the format and (optionally) the UUID version.

The RFC is very specific on how UUIDs should be formatted.  However, some systems/platforms like PostgreSQL produce or allow similar but non-compliant UUIDs which some developers may need to allow.  (Their dashes are in different spots, or they might be wrapped in curly braces, but still contain the same number of hexadecimal characters). To accommodate both parties, I've added a `strict` option to the constraint.  When enabled (default), the validator only allows UUIDs conforming to the exact RFC specs.

Comments and feedback would be greatly appreciated :)

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | symfony/symfony-docs#3619

**TODOs:**

 - [x] Gather feedback; make changes
 - [x] Ensure all tests pass
 - [ ] Get approval; have this added to the 2.5 milestone
 - [x] Submit PR for the documentation