Added new option to allow invalid SSL Certificates. Useful for testing

When testing against a non-production environment it's useful to allow self-signed SSL certificates. I've added an option for this.

```objc
SDWebImageManager *manager = [SDWebImageManager sharedManager];
[manager downloadWithURL:imageURL
    options:SDWebImageAllowInvalidSSLCertificates
    progress:NULL
    completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished)
    {
        // do something with image
    }
];
```