Provide option to disable timestamp in basename

Allows the automatic appending of a timestamp to the basename to be disabled with:

```ruby
Capybara::Screenshot.append_timestamp = false
```

The use case is that I'm using capybara-screenshot on [Nitrous.io](http://nitrous.io) (remote server) and since I can't open files or my browser programmatically from there I want to know that I can always find a screenshot of the latest failure at `http://myremoteserver.com:3000/tmp/screenshot.png` using the following settings:

```ruby
Capybara.save_and_open_page_path = 'public/tmp'
Capybara::Screenshot.append_screenshot_path = false
Capybara::Screenshot.append_timestamp = false
```