Kraken Authenticated

Unfortunately we didn't come very far, because we couldn't get the private API to work. We always get an error saying that the Signature is invalid. It works fine if there is just one parameter as in the getBalance method, but it doesn't work with multiple POST parameters. Maybe @mmazi can help here? There's a test in the MarketOrderDemo class.

One more problem: If all balances of all currencies are 0, the API returns an empty array instead of an empty object, which will throw an Exception. So basically the API either returns an array or an object, and we don't know how to handle that. This is the reason why the balance test will throw an error right now.

Example:
```json
//Non-empty result:
{
"error":[
],
"result":{
"ZEUR":"1.0539",
"XXBT":"0.4888583300"
}
}

//Empty result:
{
"error":[
],
"result": [ ] //This will throw an exception
}
```