Update deleteItem method

In the recent version of dynamodb API (2012-08-10) method `deleteItem` uses hash key value without object wrapper. Also, it does not use range key at all.
Refer to http://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_DeleteItem.html
Merge pull request #9 from FrostVenturePartners/api-2012-08-10

Update deleteItem method