OAIRepository: forcing clients to re-harvest

* Introduces a new OAI Set operation called "touch", available from the
  administrative web interface, that allows admins to force clients
  re-harvesting a particular OAI set. This is useful in case e.g.
  the format use to convert to e.g. OAI_DC has been enriched.
  (closes #1218)

Reported-by: Martin Lessmeister <mhl10@cornell.edu>
Tested-by: Tibor Simko <tibor.simko@cern.ch>