Tag release 2.0.0

The [1.0.0](https://github.com/samuel/python-erlastic/releases/tag/v1.0.0) release is tagged, but 2.0.0 [isn't](https://github.com/samuel/python-erlastic/tags). It'd be good to add a Git tag for that so that the release could be referred to when checking out the project as a submodule (etc).
