Make Generator Optional (Atom)

This patch makes the field generator optional in Atom, allowing to set
an empty string to disable the element in the same way it is disabled in
RSS already:

```python
 # disable generator element
 fg.generator('')
```

This fixes #89

feedgen/feed.py

@@ -165,7 +165,7 @@ class FeedGenerator(object):
                 uri = etree.SubElement(contrib, 'uri')
                 uri.text = c.get('uri')
 
-        if self.__atom_generator:
+        if self.__atom_generator and self.__atom_generator.get('value'):
             generator = etree.SubElement(feed, 'generator')
             generator.text = self.__atom_generator['value']
             if self.__atom_generator.get('uri'):