Remove date packing and invert date dependency

Before, each year, month, and day field used packing to store its values in the
same tag but at different positions. We then instantiated a
`CompositeDateField` to combine the different values. This lead to code
duplication in the storage styles for these fields. It also inverted the data
dependency. It's more natural to think of year, month, and day as part of a
date then as of a date as composed of these.

Now, only `DateField` class stores data in the files tag. This makes sense: One
tag, one field that accesses it. To obtain access to the year, month, and day
parts, the DateField is equipped with factories that create `DateItemField`
instances associated to a `DateField`. These descriptor allow us to get and set
parts of a date field.
Merge pull request #564 from geigerzaehler/mediafile-date

Remove date packing and invert date dependency