Add schema default/fixed value support in DOM

Added support for adding fixed/default values during XSD validation in the DOM extension.

This patch adds a second (optional) `$flags` argument to `DOMDocument::schemaValidate()` and `DOMDocument::schemaValidateSource()`, which is a bitmask of libxml schema validation context options. It also defines an additional PHP constant, `LIBXML_SCHEMA_CREATE`, corresponding to libxml's [`XML_SCHEMA_VAL_VC_I_CREATE`](http://www.xmlsoft.org/html/libxml-xmlschemas.html#xmlSchemaValidOption), which is currently the only valid flag that can be passed. When passed, the schema validation procedure will add default values from the schema when they are unspecified in the document.

The default value for the optional `$flags` parameter is `0`, which results in the current behaviour - hence merging this feature does not cause a BC-break.

See http://stackoverflow.com/q/15948847/889949 for more information.