Implement Popolo Event, so that memberships can be related to a legislative_period

[jpmckinney]

Follows from this Popolo discussion.

A very common question is "Who are the elected officials for this legislative term?" This can currently be done in PopIt by comparing the start date of a membership with external knowledge of the start and end date of a legislative term.

Popolo now has an Event class to model that external knowledge of a legislative term, and it will add a legislative_period property to Membership to relate a membership to a legislative term, which is useful in cases where you don't know precise start and end dates, but you do know the relationship between members and terms.