Improve SSL compatibility

Well, all of this started in the [WordPress support forum](http://wordpress.org/support/topic/improve-ssl-compatibility?replies=5#post-4891622), but I'm gonna repeat my description here in order to keep the pull request as much documented as I can.

I recently noticed an issue when logged-in users are forced to use SSL, but not scripts are not (i.e., `site_url()` returns https://mysite.com, but `get_stylesheet_directory_uri()` returns http://mysite.com/wp-content/themes/mytheme). In such cases, the `process_media_source()` method won't return the correct output (it will be something like http:/mysite.com/wp-content/themes/mythemestyle.css instead of /wp-content/themes/mythemestyle.css), so minification will not happen either. A typical situation could consist in a filter being applied to `site_url()`, forcing it to always use HTTPS as scheme, or having `FORCE_SSL_LOGIN` set to true in *wp-config.php*.

In my own environment, I was able to solve this issue by modifying some lines from `process_media_source`, found into *includes/class-bwp-minify.php*. More exactly, I replaced this:

```PHP
$tmp_src = str_replace(site_url(), '', $src);
```

With this:

```PHP
$site_url = site_url();

if ( 0 === strpos( $src, 'https' ) ) {
	$src = str_replace( 'https', 'http', $src );
}

if ( 0 === strpos( $site_url, 'https' ) ) {
	$site_url = str_replace( 'https', 'http', $site_url );
}

$tmp_src = str_replace( $site_url, '', $src );
```

It would be great to know if this can be added to the actual plugin, since I'm not entirely sure that this fix won't trigger any other issue.

Again, you're doing a great piece of work here :)