Added support for the "set" parameter in OAI

Added support for the `set` parameter in OAI, which allows for selecting a specific collection from the archive.

An important thing I found is that when a `resumptionToken` is send in the request, the `set` and `metadataPrefix` are not allowed to be in the parameters.

For the OAI documentation on sets see: http://www.openarchives.org/OAI/openarchivesprotocol.html#SelectiveHarvestingandSets

This feature is needed for the Universiteit Utrecht Kaarten extractor I'm currently working on.