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.
redis should report as a loaded extension

Redis has been implemented in its entirety in PHP by @sgolemon,
and there currently is no way of specifying that it should show
as a loaded extension.

This is a temporary workaround (such as the APC one) to just return true
for extension_loaded('redis') when the Redis class exists. Eventually
this should probably be done from PHP, such as done with
IMPLEMENT_DEFAULT_EXTENSION in C++.