Add key version to configuration

Hi!

I kept getting this error:
![screenshot 2013-12-19 15 57 44](https://f.cloud.github.com/assets/68570/1783135/f3cf895c-68bd-11e3-8259-2cbc37c6b590.png)

Eventually I found that it had to do with the key_version which was 2 in my case but defaults to 1 in your gem. Ofcourse it is possible to set the key_version in my controller like

``` ruby
omnikassa = OmniKassa::Request.new
omnikassa.key_version = 2
```

But I think it would be nicer to encourage users to verify that their key_version is indeed 1 by letting them specify it in their initializer. ( I know it would've saved me some time :wink: )

Thanks for making this gem!