Necessary classes to implement query builders.

## Query Parser Builders

The package `org.apache.lucene.queryParser.builders` contains the interface that builders must implement, it also contain a utility {@link org.apache.lucene.queryparser.flexible.core.builders.QueryTreeBuilder}, which walks the tree and call the Builder for each node in the tree. Builder normally convert QueryNode Object into a Lucene Query Object, and normally it's a one-to-one mapping class. But other builders implementations can by written to convert QueryNode objects to other non lucene objects.