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
 - [ ] Error handling for non-scalar values in lists
 - [ ] Uneven number of elements in a list that should be treated like a hash
 - [ ] Non-float value as a hash value
 - [ ] Error handling for null
 - [ ] README
 - [ ] Logging
And what do you think if Command can have a method like "verify" which takes all parameters? Command already takes Solr parameters to invoke a method so this seems not be be a revolution. Am I right?