Update the APIHelper mock in the tests.

Added mock functions to get a region id using it's UN_CODE and
get a country id using it's ISO3.
Update the observation helper.

An observation always refers to a region, the region can be
looked up by its UN_CODE or its ISO3 (if it is a country).
Update the observation parsing. Close #43.

An observation always refers to a region. In the XML file the
observation may specify the region UN_CODE using the <region> tag,
or it may specify the region ISO3 (if it is a country) using the
<country> tag.