Include stripe account parameter for setting header.

This PR enables you to set the stripe account that you want to send your request to, by setting the `stripe_account` header. You can now do this:

```
Stripe.stripe_account = "acct_0000"
Stripe::Charge.create({:card => {:number => '4242424242424242'}}, 'sk_test_master')
```

The above request will send a request under the account `acct_0000`. This is useful for connect customers with multiple accounts and a single master api key.