Add integration with Occurrence to Arel

[avit]

Changing to use the IceCube::Occurrence class as a wrapper for times means having to map them back to times when using with ActiveRecord & Arel:

Model.where("time IN ?", schedule.occurrences)
# => TypeError (Cannot visit IceCube::Occurrence)

Model.where("time IN ?", schedule.occurrences.map(&:to_time))
# => [#<Model id: 1, time: ...>]

It would be nice to have Occurrences work transparently as Time objects. Need to investigate further... Can we override #class without making bad stuff happen?

Adding this workaround in a rails initializer makes it work for now:

class Arel::Visitors::ToSql
  def visit_IceCube_Occurrence(value)
    quoted(value.to_time)
  end
end