Fix behaviour of DeleteItem.

The current behaviour makes DeleteItem behave like UpdateItem
using the "Delete" action. This actually does not delete an item
but an attribute.

See http://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_UpdateItem.html

This patch makes DeleteItem deleting items using the DeleteItem api
method:
http://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_DeleteItem.html
Rename attribute modifier

Renames all methods that modify attributes
add, update, delete to mAttributes which enables
other methods like DeleteItem to stay conform
with the DynamoDB API endpoints.
Merge pull request #34 from rubyphunk/master

Fix behaviour of DeleteItem.