API should return the Editors/Authors of a Pad

[JohnMcLear]

It would be nice if I could list the editors/authors of a pad via the API based on the padID.

[Pita]

Yammer did this too afaik... maybe we can take code from there

[marcelklehr]

@melicin is on this (#720), i suppose.

[marcelklehr]

Close, pls. (see #819)