Merge pull request #146 from leelynne/master

Remove pointer to Error so that dynamodb.Error impelents error correctly...
Value is optional and minimum length is 1
Merge pull request #148 from ProTip/master

Value is optional and minimum length is 1
add structures for dynamodb global secondary indexes
gsi to a query builder
query attribute name for gsi definitions
Merge pull request #152 from superduper/dynamodb_gsi

dynamodb: Add GlobalSecondaryIndex support