Adding update to instances

I have a added a simple way of updating instances of `Client`, `Offer` and `Subscription`. It creates the put request and just merges the attributes of the resulting data back to the instance. 

```ruby
client = Paymill::Client.find("client_88a388d9dd48f86c3136")
client.update(:email => "carl.client@example.com")
```
would update the email address of `client`only if it was returned in the response.