Added convenience methods for showing simple alerts with Ok button

Added methods, which allows showing UIAlertViews with only a Ok button in a single line, as easy as:

// Shows simple alert with Ok button and message only. No block is performed.
[UIAlertView simpleAlertWithText:@"Message"];

// Shows simple alert with Ok button, title and message. No block is performed.
[UIAlertView simpleAlertWithTitle:@"Title" andText:@"Message"];

// Shows simple alert with Ok button, title and message. Performs block after tapping on Ok.
[UIAlertView simpleAlertWithTitle:@"Title" text:@"Message" completionBlock:^{
    // code
}];