Use COMMAND and some helper functions for command parsing

The method before was not too robust, especially with new commands. It needed a thousand small special cases, so users don't interfer with each others database.

Using the new `COMMAND` simplifies the approach. It also made it possible to switch to [redic](https://github.com/amakawa/redic) for the Redis connection which further simplifies the parsing, formatting and namespacing try.redis has to do. This closes #29