Updated extract_features to take a leveldb/lmdb config option.
Updated Database interface to use custom KV type rather than std::pair.  Removed two buffer copies in dereference operation for DB iterators.
Added a couple of sanity checks to make sure the datum buffer sizes matched what we expected.
Added get interface to Database.  Added test cases for Database.  Fixed a few bugs related to ReadOnly mode in Database in order to pass test cases.