Add startDate and stopDate to Relationships

Relationships should support effective start and end dates.

Changes to data model:

• add relationship.start_date
• add relationship.end_date

Changes to domain objects:

• add Relationship.startDate
• add Relationship.endDate

Changes to API (needs further thought):

• add a variant of the getRelationshipsByPerson method with a Date effectiveDate parameter?
• add a variant of the getRelationships method with two Date parameters (for a range)?

Changes to UI (personRelationships portlet)

• Currently-active relationships should be shown above closed relationships
• If an existing relationship has date bounds, show them (in small text)
• Change the add relationship popup to allow an optional start date
• The [x] which currently pops up a delete function should give you the option of closing vs deleting