SimpleXML schema validate patch

Here is a patch against the latest CVS for SimpleXML to fix its schema validation support. It does the following: 1) Includes all the correct libxml2 schema header files. 2) Determines whether schema support is available. 3) Avoids refreeing the parser. 4) Inverts valid / invalid logic to work correctly. Please apply. -adam PS: Let me know if I generated the patch incorrectly.