title: DynamoDBContext with conditional update
body: <p>in my application i'm using amazon object persistence model and use DynamoDBContext for operations(CURD and execute queries). i need to know that how can i use conditional update using DynamoDBContext. and it should be asynchronous.</p>
  
  <p>I know the way of doing it using amazon low-level API.</p>
  
  <p>my application is developed using C# Dot Net(4.5)
  thank you.</p>