Shrink down the exposed methods.

The catalog really only cares right now about getting or
creating a logbook for a given job, lets keep the api simple
to just do that until further notice.

taskflow/catalog.py

@@ -24,19 +24,14 @@ class Catalog(object):
 
     __metaclass__ = abc.ABCMeta
 
-    @abc.abstractmethod
-    def fetch(self, job):
-        """Fetches a logbook for a job."""
-        raise NotImplementedError()
-
     @abc.abstractmethod
     def __contains__(self, job):
         """Checks if the given catalog has a logbook for a job."""
         raise NotImplementedError()
 
     @abc.abstractmethod
-    def create(self, job):
+    def create_or_fetch(self, job):
-        """Creates a new logbook for a job."""
+        """Creates a new logbook for a job or gives back an old one."""
         raise NotImplementedError()
 
     @abc.abstractmethod