Issue 64

Not ready for merge.

CHANGES:
- Each individual test uses its own `cdb` connection.
- Using `data.table` instead of `data.frame` for lookup tables.
- ~~In lookup table, renamed `key->keys` and `value->values` (perhaps we could find another solution to this). Since `data.table` doesn't allow for column name `key`.~~

TODO:
- [x] ~~Refactor `put_lookup` and `get_lookup` - and move them to the `cdb` class.~~
- [x] ~~Remove column names from lookup table (since it's not necessary to have `key`/`value`.~~