Add create_table and drop_table commands and generalize copy command cod...

This PR adds the new commands `create_table` and `drop_table` (while also generalizing the implementation of `copy` a little bit).

This is useful for those using this gem with cron jobs to regularly export a csv that gets updated to Redshift. The workflow for that would be to `create_table` with a certain schema, `copy` the csv, the first time, and on each following update to `drop_table` before creating it and copying the data, as an easy way to avoid duplicate rows in Redshift.

Let me know if the structure of this is okay, I am happy to make changes as you see fit!

Thanks,
Ilya