Adding attachments

Add an attachment to a UVConfig and the next UVTicket will send them across. I am not sure if this is the best way to do this bus it's working fine in my tests.

Usage examples:

```Objective-C
    UVConfig *config = ...   

    // attaching a JSON file
    NSData *jsonData = [NSJSONSerialization dataWithJSONObject:...
                                                       options:NSJSONWritingPrettyPrinted
                                                         error:nil];
    NSString *base64EncodedString = [jsonData base64EncodedStringWithOptions:0];
    [config addAttachmentNamed:@"file.json"
                   contentType:@"text/json"
             base64encodedData:base64EncodedString];


    // attaching a screenshot of a view
    UIGraphicsBeginImageContextWithOptions(view.bounds.size, NO, 0);
    [view drawViewHierarchyInRect:view.bounds afterScreenUpdates:YES];
    UIImage *snapshotImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    
    NSData *pngData = UIImagePNGRepresentation(snapshotImage);
    NSString *base64EncodedString = [pngData base64EncodedStringWithOptions:0];
    [config addAttachmentNamed:@"screenshot.png"
                   contentType:@"image/png"
             base64encodedData:base64EncodedString];
```