Fix NullPointerException - return w/ no resources

There are some cases when a call to the API will return a response body
with no resources. In this case, the Measures class will fail with a
NullPointerException because it expects to be able to fetch both
//resources/resource/date and //resources/resource/version.

This commit adds null checks for these and adds some tests for the
following methods (spelling was corrected on addMeasureFromNode(Node):

- Measures.addMeasureFromNode(Node)

- Measures.addAllMeasuresFromDocument(Document)