Bump to version 1.6.0

Version 1.6.0 adds the following bugfixes and features:

* Properly tear down subscription socket (#83)
* Implement send_tick message and tick event
* Add a timeout parameter to the main function
* Implement GET_BINDING_MODES
* Implement GET_CONFIG
* Implement GET_MARKS
* Fix pickling of types by fixing a _ReplyType exception (#89)
* Add the sticky property

setup.py

@@ -9,7 +9,7 @@
 
 setup(
     name='i3ipc',
-    version='1.5.1',
+    version='1.6.0',
     description='An improved Python library for i3wm extensions',
     long_description=long_description,
     url='https://github.com/acrisci/i3ipc-python',