[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.
Yes I was going to do it in 2 hours. Would you add it to an already existing integration test file? If so, which?