[#8] Improve resource format detection

The 'guess_resource_format' function looks for common patterns in popular
geospatial services and file extensions. It just looks at the provided URL,
it does not attempt to perform any remote check. By default, it will use the
mimetypes module if no match was found before to try to guess the format.

On the previous version, all resources in documents of type 'service' were
queried to see if they were actually WMS. This is no longer the case,
but services flagged as 'wms' can be verified if the following setting
is set to True: ckanext.spatial.harvest.validate_wms
[#8] Change 'Resource locator' string for unnamed resources

'Resource locator' was confusing, has been replaced by 'Unnamed
resource' and made translatable. Also don't set description if not
present, set name.