Integration with xml-commons resolver

Ant integration with xml-commons resolver.

These classes enhance the `<xmlcatalog>` datatype to support external catalog files using the xml-commons resolver, in accordance with the OASIS "Open Catalog" standard. They will be used if and only if the xml-commons resolver library is available on the classpath.

@see Apache xml-commons Project @see org.apache.tools.ant.types.XMLCatalog @see org.apache.tools.ant.types.resolver.ApacheCatalogResolver @see org.apache.tools.ant.types.resolver.ApacheCatalog @author Craeg Strong