A unique ID generator. Default is {@link org.archive.uid.UUIDGenerator}. To use another ID Generator, set the System Property `org.archive.uid.GeneratorFactory.generator` to point at an alternate implementation of {@link org.archive.uid.Generator}.

## TODO

- MIME boundaries have upper-bound of 70 characters total including 'blank line' (`CRLFCRLF`) and two leading hyphens. Add to {@link org.archive.uid.Generator} interface an upper-bound on generated ID length.
- Add example of an actionable uid generator: e.g. `http://archive.org/UID-SCHEME/ID` where scheme might be `UUID` and an ID might be `f9472055-fbb6-4810-90e8-68fd39e145a6;type=metadata` or, using ARK: `http://archive.org/ark:/13030/f9472055-fbb6-4810-90e8-68fd39e145a6;type=metadata`.