[core/output] Add initial version of i3bar output

Add output handler for i3bar protocol and add some tests for it. Right
now, it only support start and end.

see #23

bumblebee/output.py

@@ -0,0 +1,21 @@
+# pylint: disable=R0201
+
+"""Output classes"""
+
+import sys
+import json
+
+class I3BarOutput(object):
+    """Manage output according to the i3bar protocol"""
+    def __init__(self):
+        pass
+
+    def start(self):
+        """Print start preamble for i3bar protocol"""
+        sys.stdout.write(json.dumps({"version": 1, "click_events": True}) + "[\n")
+
+    def stop(self):
+        """Finish i3bar protocol"""
+        sys.stdout.write("]\n")
+
+# vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4