Add simple party tracking support

[djfurman]

Pull Request

Fixes #3 to add buyer and seller essential parties.

Purpose

In order to create a contract, both a buyer and a seller must be clearly identified. While these characteristics are yet to be defined, a method now exists to attach buyers and seller which can be extended as needed to ensure that this identifying of information is standardized and available in the active record.

Approach

• Used eloquent one-to-many-inverse relationships to associate a buyer and seller
• Does not address finding all related agreements from the party level
• Supports current need, but will require refactoring to make this more flexible