doc(transport.driver_base): Add docstrings

Also, renamed driver_base to base.py to be consistent with storage.

Change-Id: Iff59699a7c43341c2069823a8f39373c2d6b4760
Implements: blueprint transport-base

marconi/transport/__init__.py

@@ -1,3 +1,3 @@
 """Marconi Transport Drivers"""
 
-from marconi.transport.driver_base import DriverBase  # NOQA
+from marconi.transport.base import DriverBase  # NOQA

marconi/transport/base.py

@@ -17,10 +17,14 @@ import abc
 
 
 class DriverBase:
+    """Base class for Transport Drivers to document the expected interface."""
+
     __metaclass__ = abc.ABCMeta
 
     @abc.abstractmethod
     def listen():
-        #TODO(kgriffs): If this is all there is to DriverBase, do we
-        # even need it?
+        """
+        Called to start listening for client requests when Marconi is
+        ran in self-hosting mode.
+        """
         pass