Return URL for image used for RunKeeper Login button.
        
        @param button_color:  Button color. Either 'blue', 'grey' or 'black'.
                              Default: 'blue'.
        @param caption_color: Button text color. Either 'white' or 'black'.
                              Default: 'white'
        @param button_size:   Button width in pixels. Either 200, 300 or 600.
                              Default: 200
        @return:              URL for Login Button Image.

def get_login_button_url(self, button_color=None, caption_color=None, button_size=None):
        """Return URL for image used for RunKeeper Login button.
        
        @param button_color:  Button color. Either 'blue', 'grey' or 'black'.
                              Default: 'blue'.
        @param caption_color: Button text color. Either 'white' or 'black'.
                              Default: 'white'
        @param button_size:   Button width in pixels. Either 200, 300 or 600.
                              Default: 200
        @return:              URL for Login Button Image.
        
        """
        if not button_color in settings.LOGIN_BUTTON_COLORS:
            button_color = settings.LOGIN_BUTTON_COLORS[0]
        if not caption_color in settings.LOGIN_BUTTON_CAPTION_COLORS:
            caption_color = settings.LOGIN_BUTTON_CAPTION_COLORS[0]
        if settings.LOGIN_BUTTON_SIZES.has_key(button_size):
            button_size = settings.LOGIN_BUTTON_SIZES[button_size]
        else:
            button_size = settings.LOGIN_BUTTON_SIZES['None']
        return settings.LOGIN_BUTTON_URL % (button_color, 
                                            caption_color, 
                                            button_size)