Bug 6440: Implement OAI-PMH Sets

Bug 6440: Implement OAI-PMH Sets

New sql tables:
  - oai_sets: contains the list of sets, described by a spec and a name
  - oai_sets_descriptions: contains a list of descriptions for each set
  - oai_sets_mappings: conditions on marc fields to match for biblio to be
    in a set
  - oai_sets_biblios: list of biblionumbers for each set

New admin page: allow to configure sets:
  - Creation, deletion, modification of spec, name and descriptions
  - Define mappings which will be used for building oai sets

Implements OAI Sets in opac/oai.pl:
  - ListSets, ListIdentifiers, ListRecords, GetRecord

New script misc/migration_tools/build_oai_sets.pl:
  - Retrieve marcxml from all biblios and test if they belong to defined
    sets. The oai_sets_biblios table is then updated accordingly

New system preference OAI-PMH:AutoUpdateSets. If on, update sets
automatically when a biblio is created or updated.

Use OPACBaseURL in oai_dc xslt