* The 'filter' method on collections now returns another
collection.
* Add a 'memberships' property to Event, primarily so that
members of a legislative period can easily be found.
* Add 'elections' and 'legislative_periods' properties to
EventCollection to filter down the events to just
elections or terms.
* Add helper methods to the `Popolo` class to access
elections, terms, the latest term, etc. directly.
* Thanks to Andy Lulham for contributing these improvements
in this version.