initial version of simplexml refactor
add the saveXML alias
Add an option to match namespaces when not specifying one.

This restores functionality present in HHVM's previously "broken"
SimpleXML implementation but apparently used throughout Facebook's code.

I wasn't sure on where to put the config option, so I created
a separate SimpleXML node just in case we needed more of these in the
future.

Don't forget to -vSimpleXML.EmptyNamespaceMatchesAll=true to enable
this.