Changed the convention so that all NSValueTransformers used by MTLXMLAdapter accept a NSArray of DDXMLNode objects. Also updated NSValueTransformer+MTLXMLTransformerAdditions to follow this convention in addition to several helper functions for parsing integers, NSURLs and NSDate objects.
The default case if no NSValueTransformer has been specified, is to get the text of the first xml node returned by the specified xpath, just as before.
This allows us to handle arrays of elements as selected by an xpath expressions without special case code. However now all code assuming a single xmlnode has to explicitly get the first element.

This breaks backwards compatibility, so bumping up to version 0.2.0