Issue title: Support for deleting discounts
 Issue body: ``` javascript
delete_discount: function(customer_id, cb) {
   del("/v1/customers" + customer_id + '/discount', {}, cb);
}
```

 Comments: 
 Comment 0: A pull request would be welcome; ideally with a README.md and test updates, too.

 Comment 1: Should be fixed in latest version :).

{'source': 'GithubIssues', 'repo_name_with_owner': 'stripe/stripe-node', 'issue_id': 12300717, 'repo_stars': 3138.0}