Make owner() and group() methods, rather than properties.

It didn't make sense to keep them as properties as all other metadata-querying APIs
are now exposed under the form of methods (e.g. is_dir(), etc.), and there's no
stat() caching anymore.

pep-0428.txt

@@ -594,12 +594,12 @@ Higher-level methods help examine the kind of the file::
     False
 
 The file owner and group names (rather than numeric ids) are queried
-through matching properties::
+through corresponding methods::
 
     >>> p = Path('/etc/shadow')
-    >>> p.owner
+    >>> p.owner()
     'root'
-    >>> p.group
+    >>> p.group()
     'shadow'