Use GSIs in FakeDynamo when the IndexName has two dashes

The existing FakeDynamo code doesn't make use of a GSI if the
range key of the GSI is the hash key of the table. This makes it
impossible to test queries against GSI where this is the case. As
a minimal addition, this patch explicitly uses the GSI path if
the indexName starts with 'gsi'.
Merge pull request #53 from Medium/jamie-gsi-with-table-hash-key-as-range-key

Use GSIs in FakeDynamo when the IndexName has two dashes.