Adding gsi and lsi functionality

I'm not sure this is complete, but I thought someone might find it useful.

This PR adds support for GlobalSecondaryIndexes & LocalSecondaryIndexes to the dynamo db feature.

To achieve this i had to make the KeySchema section comply with the current [spec](http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-dynamodb-table.html#cfn-dynamodb-table-keyschema).