Bug fix: Incorrect messages displayed in "Export settings" section.

Incorrect messages are shown in "Export settings" section:

- "Existing settings will be overwritten" displayed even if the browser's localStorage doesn't contains any previously saved settings.
![existing_settings](https://cloud.githubusercontent.com/assets/6863616/2615233/bfd01270-bbf6-11e3-815f-b43580906d59.png)

- "This feature is not supported by your web browser" is displayed even if browser supports localStorage.
![not_supported](https://cloud.githubusercontent.com/assets/6863616/2615240/e93cbf46-bbf6-11e3-8664-fe1c229b3710.png)