Searcher: add skeleton
move search routine to Search class
add some document statistics method to indexer
Searcher: implement very basic tf-idf
Searcher: search method return as list of Result class
send score to client
Searcher: use double compare method
Merge branch 'searcher' into indexer
make collection abstract class
Database: can create in-memory database
implement remove document
DefaultIndexer: add override annotation