Data entries should include more metadata

[igneus]

• build lib/calendarium-romanum/data.rb by a rake script from the data files in data/ instead of having it coded by hand
• include metadata from the "YAML front matter" of each file in the respective SanctoraleFile instance

The point of having CalendariumRomanum::Data is to make the packaged data files easily accessible, but currently the amount of information available without actually looking in each file is very limited and rather useless.