Big refactor: flask, peewee, tests, structure changes and more

This is a big refactor of the re:dash backend that should improve stability and code maintenance.

Some brief overview of the changes:
1. `rd_service` was renamed to `redash`.
2. `rd_service/cli.py` is now `manage.py` and it's a full CLI to control re:dash (start server/workers, shell, create/drop tables and more in the future).
3. We no longer use `Tornado` but rather `Flask`. Also for re:dash's own data, we use [peewee](http://peewee.readthedocs.org/en/latest/) ORM instead of Django ORM.
4. There are now tests for the backend code (under `tests`). It's not a full coverage (~70%), but it's a start.
5. Instead of plain API keys authentication (for CSV endpoint) we now use HMAC.
6. Version information is now at `redash.__version__` and we will try to follow [SemVer](semver.org).

To run the tests, you simply run `make test`. 
For `make test` to work, you need: `nose` and `coverage` (you can just `pip install -r dev_requirements.txt`).