Adds .insertBytes(...) to NSMutableData in Swift

NSMutableData.insertBytes(bytes: withLength: atIndex: )

public extension NSMutableData {
    
    func insertBytes(bytes: UnsafePointer<Void>, withLength length: Int, atIndex index: Int) -> NSMutableData {
        
        if index > self.length {
            return NSMutableData()
        }
        
        var data = self // Holds self for modification
        
        let lowerRange = NSMakeRange(0, index)
        let upperRange = NSMakeRange(index, data.length-index)
        
        let lowerHalfOfData = data.subdataWithRange(lowerRange)
        let upperHalfOfData = data.subdataWithRange(upperRange)
        
        data = NSMutableData()
        data.appendData(lowerHalfOfData)
        data.appendBytes(bytes, length: length)
        data.appendData(upperHalfOfData)
        
        return data
    }
}