Enable to use preview both SSL site and non-SSL site

Hi embed.ly team,

We use jquery.preview.js on SSL site, but this javascript uses HTTP request inner script as following:

```javascript
ajax : function(data){
  // Make the request to Embedly. Note we are using the
  // preview endpoint: http://embed.ly/docs/endpoints/1/preview
  $.ajax({
    url: 'http://api.embed.ly/1/preview',
    dataType: 'jsonp',
    data: data,
    success: this._callback,
    error: this.error
  });
}
```

This causes a browser error. (e.g. [blocked] The page at https://example.com/ ran insecure content from ...)

So in this PR, I have changed the request url from HTTP only to using location.protocol.