Add support for using a url as the source

### What is the problem / feature ?

It'd be nice to be able to use `flex.core.load` with the url of a swagger schema.

### How did it get fixed / implemented ?

Added support for using a url.

### How can someone test / see it ?

Pass a url into the function and see that you get back the parsed and validated swagger schema.

*Here is a cute animal picture for your troubles...*

![raccoon_cute_pose](https://cloud.githubusercontent.com/assets/824194/4913179/f8309410-64ab-11e4-99b8-ca40549490b0.jpg)