Add time_diff

Ability to modify oauth_timestamp. Usefull for slower uploads. With Typo-fix wich occurred in pull request #53

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"
```
Here my upload of ~190MB
```
user@host ~/ $ time ruby upload 
/home/user/flickraw/lib/flickraw/oauth.rb:156:in `post': timestamp_refused (FlickRaw::OAuthClient::FailedResponse)
	from /home/user/flickraw/lib/flickraw/oauth.rb:96:in `post_multipart'
	from /home/user/flickraw/lib/flickraw/api.rb:147:in `upload_flickr'
	from /home/user/flickraw/lib/flickraw/api.rb:93:in `upload_photo'
	from upload:28:in `<main>'

real	35m25.625s
user	0m0.224s
sys	0m0.467s
user@host ~/ $ vim upload # Setting flickr.time_diff to 30*60
user@host ~/ $ time ruby upload 

real	30m48.949s
user	0m0.231s
sys	0m0.387s
```