Wraps superagent requests in Bluebird promises.
e.g
require('superagent').get('http://api.mysite.com/?id=23432').use(require('superagent-promises')).end();
superagent-promises caches the native .end()
method of superagent. For testing purposes an .uncache()
method is provided, which should be called immediately before spying or stubbing superagent.end()