Added simple web server which shows current state of postgres

Server always returns json which contains some status of postgres:
* instance type: master/slave
* xlog location for master
* xlog received_location, replayed_location
This server could be used by haproxy
GET / returns 200 if there is postgres master behind governor.
GET /slave returns 200 is there is postgres slave behind governor
In all other cases it returns 503
Currently server always listening on 0.0.0.0:8080, but it should be
configurable. In the next versions this rest api could also report some
status of etcd and could be used by is_healthiest_node method

tests/test_api.py

@@ -0,0 +1,66 @@
+import psycopg2
+import sys
+import unittest
+
+from helpers.api import RestApiHandler, RestApiServer
+from test_postgresql import psycopg2_connect
+
+if sys.hexversion >= 0x03000000:
+    from io import BytesIO as IO
+else:
+    from StringIO import StringIO as IO
+
+
+def false(*args, **kwargs):
+    return False
+
+
+def throws(*args, **kwargs):
+    raise psycopg2.OperationalError()
+
+
+class MockPostgresql:
+
+    def connection(self):
+        return psycopg2_connect()
+
+    def is_running(self):
+        return True
+
+
+class MockGovernor:
+
+    def __init__(self):
+        self.postgresql = MockPostgresql()
+
+
+class MockRequest:
+
+    def __init__(self, path):
+        self.path = path
+
+    def makefile(self, *args, **kwargs):
+        return IO(self.path)
+
+
+class MockRestApiServer(RestApiServer):
+
+    def __init__(self, Handler, path, *args):
+        self.governor = MockGovernor()
+        if len(args) > 0:
+            self.governor.postgresql.is_running = args[0]
+        self._cursor_holder = None
+        Handler(MockRequest(path), ('0.0.0.0', 8080), self)
+
+
+class TestRestApiHandler(unittest.TestCase):
+
+    def __init__(self, method_name='runTest'):
+        super(TestRestApiHandler, self).__init__(method_name)
+
+    def test_do_GET(self):
+        MockRestApiServer(RestApiHandler, b'GET /')
+        MockRestApiServer(RestApiHandler, b'GET /', throws)
+
+    def test_get_postgresql_status(self):
+        MockRestApiServer(RestApiHandler, b'GET /', false)