Adds a ConfigParserProperty class

This adds a ConfigParserProperty class to allow binding and controlling a ConfigParser values from other properties. See the class doc for details.

An example that goes with it is below. You'll notice that after typing stuff and closing and restarting kivy, the textinputs will be auto fileed with the config values.

```python
    from kivy.lang import Builder
    from kivy.app import runTouchApp
    from kivy.uix.label import Label
    from kivy.properties import ConfigParserProperty
    from kivy.config import ConfigParser
    import os
    
    
    class UserInfo(Label):
    
        name = ConfigParserProperty('', 'info', 'name')
        address = ConfigParserProperty('', 'info', 'address')
    
        def __init__(self, **kw):
            super(UserInfo, self).__init__(**kw)
            if not os.path.exists('info.ini'):
                with open('info.ini', 'w'):
                    pass
            config = ConfigParser()
            config.read('info.ini')
            self.name = config
            self.address = config
    
    
    kv = '''
    BoxLayout:
        orientation: 'vertical'
        GridLayout:
            cols: 2
            TextInput:
                on_text: info.name = self.text
                text: info.name
            TextInput:
                on_text: info.address = self.text
                text: info.address
        UserInfo:
            id: info
            text: 'Name: {}\\nAddress: {}'.format(self.name, self.address)
    '''
    
    runTouchApp(Builder.load_string(kv))
```