ProxyWebView segfaults on application exit.

The following testcase demonstrates the issue. The test will segfault on windows 10 pyside 1.2.2 and python 2.7.13

```python
import unittest
import faulthandler

from jigna.qt import QtCore, QtGui
from jigna.core.proxy_qwebview import ProxyQWebView

faulthandler.enable()

class TestProxyQWebview(unittest.TestCase):

def setUp(self):
self.app = QtGui.QApplication.instance() or QtGui.QApplication([])

def test_setUrl(self):
app = self.app

def create():
widget = ProxyQWebView()
widget.show()
widget.setUrl(QtCore.QUrl('https://www.enthought.com'))
print 'DONE'

QtCore.QTimer.singleShot(10, create)
QtCore.QTimer.singleShot(10000, app.quit)
print 'Start'
app.exec_()
```