Added QueryResult class.

This class represents the results of a query() operation of an
ExpressionEngine. Results can be both nodes and attributes.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1563458 13f79535-47bb-0310-9956-ffa450edef68
Reworked ExpressionEngine interface.

An expression engine can now deal with nodes of different types; a NodeHandler
for the type desired has to be passed when invoking a method. A few methods
have been added for generating specific kinds of keys.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1563459 13f79535-47bb-0310-9956-ffa450edef68