Implement basic DT get / set

This implements the very basics of data-type get & set. The protocol
buffer wrappers for the various data types are not included in this
commit.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Add convenience wrappers for the Counter data-type

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>