Added nodeForXpath:namespaceMapping:error method

Hello there,

I'm writing an iOS application parsing some XML with many namespaces and I've been looking very often for single nodes in my XML documents.

I was told to use the method ```nodesForXpath:namespaceMappings:error:``` method to work with namespaces, and I found out the ```nodeForXpath:error:``` method not resolving the namespaces but returning a single node instead of an array. So I ended up writing a similar method but resolving namespaces as well.

I also took the liberty of changing the ```@implementation``` statement to match the one in the header file and in the filenames.

I'm completely new to Objective-C so please tell me if I did something wrong.

Regards.