Allow title to be unset for pages

According to a test case provided in #22 it is possible
for the title of a confluence page to be unset.

Refs #22

confluence/models/content.py

@@ -66,10 +66,12 @@ def __init__(self, json):  # type: (Dict[str, Any]) -> None
         if id.startswith('att'):
             id = id[3:]
         self.id = int(id)
-        self.title = json['title']  # type: str
         self.status = ContentStatus(json['status'])  # type: ContentStatus
         self.type = ContentType(json['type'])  # type: ContentType
 
+        if 'title' in json:
+            self.title = json['title']  # type: str
+
         if 'metadata' in json:
             self.metadata = json['metadata']  # type: Dict[str, Any]