Use attrs, to remove verbose __init__ and __repr__ methods

Backwards compatibility is strictly preserved in `__init__`, including parameter names, defaults and position. Whenever that's difficult using `attrs`, the custom `__init__` is kept instead (for the time being).

`__repr__` methods have changed to the format `attrs` use, but people don't rely on this for anything other than debug output, so it shouldn't be a problem.

fbchat/_page.py

@@ -1,22 +1,24 @@
 # -*- coding: UTF-8 -*-
 from __future__ import unicode_literals
 
+import attr
 from ._thread import ThreadType, Thread
 
 
+@attr.s(cmp=False, init=False)
 class Page(Thread):
     """Represents a Facebook page. Inherits `Thread`"""
 
     #: The page's custom url
-    url = None
+    url = attr.ib(None)
     #: The name of the page's location city
-    city = None
+    city = attr.ib(None)
     #: Amount of likes the page has
-    likes = None
+    likes = attr.ib(None)
     #: Some extra information about the page
-    sub_title = None
+    sub_title = attr.ib(None)
     #: The page's category
-    category = None
+    category = attr.ib(None)
 
     def __init__(
         self,