Extended search engine API with a possibility to pass full datetime values d1, d2 in the format "YYYY-MM-DD hh:mm:ss" to retrieve records created or modified in a certain time interval. Useful e.g. for OAI interface client code. Note that d1, d2 take precedence over (d1y, d1m, d1d), (d2y, d2m, d2d) tuples if they exist. Added some more test cases.