CreateIndex should return an Index instance

[eskombro]

In the current implementation, getIndex() returns an Index instance, but createIndex() returns the response of the POST request for creating an index. This forces a usage that looks like this:

...
String i = ms.createIndex(indexUid);
Index index = ms.getIndex(indexUid);
index.addDocuments(...)
...

In case createIndex succeeds, it should return an Index instance which simplifies the usage to

...
Index index = ms.createIndex(indexUid);
index.addDocuments(...)
...

In case of error, this will be handled by #7