Card 608 Client side fix for binary data null saves

Example:

```js
StackMob.Model.extend({ binaryFields: ['pic', 'largepic'] });
```
On-save, the `pic` and `largepic` fields will not be sent up to the server if they start with "http", implying that it's already a binary field that's been saved to S3
Adding binaryField specs
Making the spec more reusable
Add more than one binaryField and add https
Merge pull request #53 from stackmob/b/608-binaryfield-fix

Card 608 Client side fix for binary data null saves