Adds prefix to jsonp callbacks

- Prevents collision with libraries using the default prefix
- See duojs/test#69

Signed-off-by: Christopher Rogers <chrissrogers@gmail.com>

lib/recurly.js

@@ -235,7 +235,7 @@ Recurly.prototype.jsonp = function (route, data, done) {
 
   var url = this.url(route) + '?' + qs.stringify(data);
 
-  jsonp(url, { timeout: this.config.timeout }, function (err, res) {
+  jsonp(url, { timeout: this.config.timeout, prefix: '__rjs' }, function (err, res) {
     if (err) return done(err);
     if (res.error) {
       done(errors('api-error', res.error));