EVAL/EVALSHA support

This PR ads support for `EVAL`/`EVALSHA`. Please note that this is a very early PR to get feedback as soon as possible. A lot of stuff needs to be done. I will add a bunch of questions inline.

## TODOs
 - [x] Move parameter parsing to commands
 - [x] Error handling for non-scalar values in lists
 - [x] Uneven number of elements in a list that should be treated like a hash
 - [x] Non-float value as a hash value
 - [x] Error handling for null
 - [x] README
 - [x] Logging