A bit of fefactoring of the AdlibExtractor:

- Removed unnecessary references to OAI
- Added defaults for query ('all'), xmltype ('unstructured') and limit
  (50) attributes, this should simplefy configuration of new sources
- Only allow setting specific 'adlib_*' settings in the
  source_defenition
- Adlib seems to return valid XML, removed the hacky parse function
- Simplified paginating through search results