Add support for listing keys in a specific S3 bucket

Includes support for specifying a `prefix`, `marker`, and/or `max_keys`. These three options are optional. `max_keys` does default to 1000. I chose to use `get_all_keys` over `list` for the ability to specify `max_keys`.

The main part of the response body is a list of key names (strings).

This is my first foray into ansible and Python so if I'm breaking conventions please let me know.