Restrict usernames with all numbers

[MichealSBingham]

Add this extension to String in order to test if a string contains only numbers.

  var isNumber : Bool {
    get{
        return !self.isEmpty && self.rangeOfCharacter(from: CharacterSet.decimalDigits.inverted) == nil
    }
}

Add this code to func validate() from extension

   else if self.isNumber{
        let message = "Usernames cannot contain all numbers."
        completion(false, message)
    }

[MichealSBingham]

Done.