[MODEL] Added dynamic getter/setter (block/proc) for MyModel.index_name

Fixes #155.

The following changes could also be applied to elasticsearch-persistence (I guess) if you want me to do it.

The tests are not very easy to read... I am taking recommendations. I have also signed the CLA.
I'd like to have an integration test for this as well, just to be on a safe side. This is potentially a feature with high invasive potential.