Add max_memory and max_memory_human to INFO command

This pull request adds `max_memory` and `max_memory_human` to the `INFO` command's output.

Redis hosting providers often disable or rename the `CONFIG` command to lock down the Redis instance to a specific configuration based on a customer's plan. This makes it difficult to programmatically get the actual max memory (and subsequently the memory utilization) of the Redis instance.

Addresses #355