Add support for Global Secondary Indexes in some missing places

As per subject

Tests complete fine, i cp'ed them into a copy of your dir structure to test run them (due to imports etc), sha1's match the PR file versions:

```
blah@blah:/git/dynamo-test/src/github.com/smugmug/godynamo/endpoints/describe_table$ sha1sum *
e0da623906bdf390ce61129bc86ed07b0a4cae3d  describe_table.go
f7099b5b6f7f8532aa2a836bf0bb1151450a0cc9  describe_table_test.go
blah@blah:/git/dynamo-test/src/github.com/smugmug/godynamo/endpoints/describe_table$ go test
PASS
ok  	github.com/smugmug/godynamo/endpoints/describe_table	0.007s
blah@blah:/git/dynamo-test/src/github.com/smugmug/godynamo/endpoints/describe_table$ cd ../create_table/
blah@blah:/git/dynamo-test/src/github.com/smugmug/godynamo/endpoints/create_table$ sha1sum *
f4c2b2d1285825834ddc08b38466a35a53a2579c  create_table.go
11cace77f92edbb1e55629a3b78effe28afe27d3  create_table_test.go
blah@blah:/git/dynamo-test/src/github.com/smugmug/godynamo/endpoints/create_table$ go test
PASS
ok  	github.com/smugmug/godynamo/endpoints/create_table	0.006s
```