L3 Validation update, building L3 without a network connection

The validation tests had an implicit precondition of schema validation, 
however there was not a mechanism to assert that.  I've added another 
allowable value ("schemaValidation") to implementationAttribute to force 
schemaValidation.  The Java support files use attribute names defined in 
JAXP 1.2 to enable schema validation.

I added the schema validation assertion to all the validation tests, 
added <assertNotNull> on return values to provide better diagnostic 
information than NullPointerException and eliminate unnecessary explict 
cast operations.

I attempted to run the validation suite against the last public Oracle 
beta, however it resulted in almost total test failure.  I would 
appreciate a report if the situation is different the current 
development version or any other implementation.

Meghnan also complained that it was not possible to build the test 
suites without a network connection.  If you invoke Ant with a 
-Dno-network parameter, the build process should allow the build process 
to run with previously downloaded DOM specs.

One of the changes performed in the spec is to remove the 
http://www.w3.... from the xml-spec DTD in the xml-source.xml files.  It 
might be good to change those to relative references and pack the 
xmlspec DTD in the spec zip files.