UUID field for Card and Stack class instances

[nelsonni]

The id field defined in the Card and Stack classes is not guaranteed to be a unique ID and is instead used to manage the set of Card or Stack instances currently placed on a Canvas. In order to provide a universally unique identifier (uuid), a new uuid field (using the uuid package) must be added to the Card and Stack class definitions in order to have a truly unique ID to identify individual Card and Stack instances across multiple IDE sessions, multiple Canvas instances, and aggregated logs from multiple systems.

[nelsonni]

Commit 7953d37 introduces UUID field for Card class.

[nelsonni]

Commit 839ab85 introduces UUID field for Stack class.

[nelsonni]

Both Card and Stack implementations of UUID fields were merged into master branch via PR #13.