[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.
Yeah I wanted to test the both the Proc argument and the block implementation since it's just a bit different... but I can remove it if you want.