Add style for QPushButton:checked

and update example

Fix #26

example/example_pyqt5.py

@@ -32,7 +32,7 @@ Requirements:
     - Python 2 or Python 3
     - PyQt4
 
-.. note.. :: qdarkstyle does not have to be installed to run 
+.. note.. :: qdarkstyle does not have to be installed to run
     the example
 
 """
@@ -55,7 +55,7 @@ def main():
     # create the application and the main window
     app = QtWidgets.QApplication(sys.argv)
     window = QtWidgets.QMainWindow()
-    
+
     # setup ui
     ui = example_ui.Ui_MainWindow()
     ui.setupUi(window)