Added a relational data store using the sequel gem.

Tests use in-memory Sqlite3

Considerable test refactoring, as the sequel gem
does not respond to

  Thing.new(occurrences: [first, second])

Rather, it uses

  a = Thing.create
  a.add_occurrence(first)
  a.add_occurrence(second)
Refactored some Ruby to use SQL more elegantly,
which fixed a bug in the TodayVerb model