added a new namespaceAware() deserializer function

[staabm]

didnt found the comments in which we discussed this topic therefore starting from scratch.
hopefully all points are adressed.

[staabm]

feel free to propose a better name.. couldnt get better then this right now

[evert]

I think I want to change the behavior of the function a little bit. Instead of :

Sabre\Xml\Deserialize\namespaceAware, I think I just want to call it Sabre\Xml\Deserializer\keyValue.

By default I want to give it the same behavior as Sabre\Xml\Element\KeyValue, but if you specify the $namespace argument (which will be optional) it will strip the namespace, but only from the elements within that namespace, and leave the others intact.

So basically, hold on a second. I will take your PR and use it as a basis for this =)

[staabm]

Sounds great!

[evert]

Oh I didn't realize that in your PR you're already retaining other elements outside of the specified namespace. Really awesome. In that case I'm only changing the function name and docblocks ;)