Fix DeprecationWarnings in Widget

As of 2.6, object.__init__ takes no arguments. Passing it arguments triggers a
DeprecationWarning or worse. Kivy startup was producing a flood of DeprecationWarnings that gave me a headache :smile:

`__init__` in `EventDispatcher` was passing kwargs to `object.__init__`; this patch removes that.

## Before:
```
...
[DEBUG  ] [ImageImageIO] Load </usr/local/lib/python2.7/site-packages/kivy/data/../data/images/defaulttheme-0.png>
 /usr/local/lib/python2.7/site-packages/kivy/uix/widget.py:136: DeprecationWarning: object.__init__() takes no parameters
   super(Widget, self).__init__(**kwargs)
 /usr/local/lib/python2.7/site-packages/kivy/uix/widget.py:136: DeprecationWarning: object.__init__() takes no parameters
   super(Widget, self).__init__(**kwargs)
 /usr/local/lib/python2.7/site-packages/kivy/uix/widget.py:136: DeprecationWarning: object.__init__() takes no parameters
   super(Widget, self).__init__(**kwargs)
 /usr/local/lib/python2.7/site-packages/kivy/uix/widget.py:136: DeprecationWarning: object.__init__() takes no parameters
   super(Widget, self).__init__(**kwargs)
 /usr/local/lib/python2.7/site-packages/kivy/uix/widget.py:136: DeprecationWarning: object.__init__() takes no parameters
   super(Widget, self).__init__(**kwargs)
 /usr/local/lib/python2.7/site-packages/kivy/uix/widget.py:136: DeprecationWarning: object.__init__() takes no parameters
   super(Widget, self).__init__(**kwargs)
 /usr/local/lib/python2.7/site-packages/kivy/uix/widget.py:136: DeprecationWarning: object.__init__() takes no parameters
   super(Widget, self).__init__(**kwargs)
 /usr/local/lib/python2.7/site-packages/kivy/uix/widget.py:136: DeprecationWarning: object.__init__() takes no parameters
   super(Widget, self).__init__(**kwargs)
 /usr/local/lib/python2.7/site-packages/kivy/uix/widget.py:136: DeprecationWarning: object.__init__() takes no parameters
   super(Widget, self).__init__(**kwargs)
 /usr/local/lib/python2.7/site-packages/kivy/uix/widget.py:136: DeprecationWarning: object.__init__() takes no parameters
   super(Widget, self).__init__(**kwargs)
 /usr/local/lib/python2.7/site-packages/kivy/uix/widget.py:136: DeprecationWarning: object.__init__() takes no parameters
   super(Widget, self).__init__(**kwargs)
 /usr/local/lib/python2.7/site-packages/kivy/uix/widget.py:136: DeprecationWarning: object.__init__() takes no parameters
   super(Widget, self).__init__(**kwargs)
[DEBUG  ] [Cache       ] register <kv.loader> with limit=500, timeout=60s
...
```

## After:
```
...
[DEBUG  ] [ImageImageIO] Load </usr/local/lib/python2.7/site-packages/kivy/data/../data/images/defaulttheme-0.png>
[DEBUG  ] [Cache       ] register <kv.loader> with limit=500, timeout=60s
...
```