Preparing for Stripe Connect support

Adding support for Stripe connect by having options parameter to each method. When options object is present with api_key set, it will use that api_key as opposed to the global api_key upon making request.

For example,

```javascript
var stripe = require('stripe')(<global_api_key>);
...
stripe.charges.create({ amount: 1000, card: stripeToken, ... },
    { api_key: <user's_api_key> }, function(err, res) {
  ...
})
...
```

@abh, what do you think?