Add support for 'connection' concept

This is a large refactor and as small as I could feasibly make it
while keeping the tests working. I'll do the documentation and
touch ups in the next commit to make digesting easier.

Change-Id: Iac5083996a183d1d8a9b6cb8f70836f7c39ee910

zuul/connection/__init__.py

@@ -0,0 +1,53 @@
+# Copyright 2014 Rackspace Australia
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may
+# not use this file except in compliance with the License. You may obtain
+# a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations
+# under the License.
+
+import abc
+
+import six
+
+
+@six.add_metaclass(abc.ABCMeta)
+class BaseConnection(object):
+    """Base class for connections.
+
+    A connection is a shared object that sources, triggers and reporters can
+    use to speak with a remote API without needing to establish a new
+    connection each time or without having to authenticate each time.
+
+    Multiple instances of the same connection may exist with different
+    credentials, for example, thus allowing for different pipelines to operate
+    on different Gerrit installations or post back as a different user etc.
+
+    Connections can implement their own public methods. Required connection
+    methods are validated by the {trigger, source, reporter} they are loaded
+    into. For example, a trigger will likely require some kind of query method
+    while a reporter may need a review method."""
+
+    def __init__(self, connection_name, connection_config):
+        # connection_name is the name given to this connection in zuul.ini
+        # connection_config is a dictionary of config_section from zuul.ini for
+        # this connection.
+        # __init__ shouldn't make the actual connection in case this connection
+        # isn't used in the layout.
+        self.connection_name = connection_name
+        self.connection_config = connection_config
+
+    def onLoad(self):
+        pass
+
+    def onStop(self):
+        pass
+
+    def registerScheduler(self, sched):
+        self.sched = sched