Remote assets beginning with // fail to load

When loading remote assets using just `//` instead of `http(s)://` causes the following issue

```
ErrorException [ Warning ]: file_get_contents(//fonts.googleapis.com/css?family=Open+Sans:300,400,600): failed to open stream: No such file or directory
```

Adding the following code to `load_file()` resolves the problem.

```
if (strpos($filename, '//') === 0)
{
$protocol = \Input::server('https', false) ? 'https:' : 'http:';
$filename = $protocol.$filename;
}
```

Start at the load_file() entry point and reproduce the failure with the protocol-relative Google Fonts URL shown in the issue. Verify that protocol-relative remote assets load correctly under both HTTP and HTTPS, while ordinary asset URLs continue to work.

Written by the indexing model from the issue text.