Added tree walker for parsing an rxm.

Delta	25 lines added, 1 line removed, 24-line increase

source/java/com/whitemagicsoftware/rxm/Parser.java

		import java.nio.file.Paths;

		+import org.antlr.v4.runtime.ANTLRInputStream;
		+import org.antlr.v4.runtime.CommonTokenStream;
		+
		+import org.antlr.v4.runtime.tree.ParseTreeWalker;
		+
		+import com.whitemagicsoftware.rxm.grammar.*;
		+
		/**
		* Parses the expression into an abstract tree.
		*/
		-public class Parser {
		+public class Parser extends QueryBaseListener {
		/**
		* Creates a Parser instance that parses a given <b>rxm</b> string.
		*/
		public Parser( String rxm ) throws ParserException {
		+ QueryLexer lexer = new QueryLexer( new ANTLRInputStream( rxm ) );
		+ CommonTokenStream tokens = new CommonTokenStream( lexer );
		+
		+ QueryParser parser = new QueryParser( tokens );
		+ QueryParser.QueryContext context = parser.query();
		+
		+ ParseTreeWalker walker = new ParseTreeWalker();
		+ walker.walk( this, context );
		+ }
		+
		+ @Override
		+ public void enterQuery( QueryParser.QueryContext context ) {
		+ System.out.println( "Enter Query" );
		+ }
		+
		+ public void exitQuery( QueryParser.QueryContext context ) {
		+ System.out.println( "Exit Query" );
		}