Add time_diff

Ability to modify oauth_timestamp. Usefull for slower uploads.

If you'r upload is to slow, oaut_timestamp will be refused on big files. So you will now be able to set a timestamp in the furture to avoid this problem:

```ruby
flickr.time_diff = 30*60 # 30 minutes
flickr.upload_photo PHOTO_PATH, :title => "Title", :description => "This is the description"
```