mts/meta: Create functions for encoding/decoding to a plain string.

[Scott Barnard]

Prerequisite for vidgrind issues #41 and #42.

Fetching, storing and modifying metadata in vidgrind is something that I am implementing currently. In iotds, MtsMedia metadata is stored as a string. The meta.Data type is useful for working with metadata, so it would be good if it was possible to convert between it and plain strings.