Implement FOREIGN KEY constraints (#237)

Implemented in this PR:
- Add support for `FOREIGN KEY` constraints in `CREATE TABLE`.
- Add support for `REFECENCES` clause in `CREATE TABLE` column
definitions.
- Add support for `ADD FOREIGN KEY` clause in `ALTER TABLE`.
- Add support for `DROP FOREIGN KEY` clause in `ALTER TABLE`.
- Add support for `DROP CONSTRAINT` clause in `ALTER TABLE`.
- Add support for `DROP PRIMARY KEY` clause in `ALTER TABLE`.

Closes
#236.
Fixes
#229.

Author: JanJakes