Basic commands use IPFS Core API

[ghost]

Most of the "business logic" buried in the core/commands package should use the IPFS Core API. We'll start with the basic commands add, cat, get, ls, and refs. The rest will follow.

Dependencies:

• Gateway uses minimal IPFS Core API #2876 Gateway uses minimal IPFS Core API
  • for the basic Core API imeplementation

[kevina]

Any rewrite of the add command is likely to cause me grief with the filestore code (#2634). I would like to be involved in any rewrite efforts. I might even be willing to just write the new code for add once I know the issues involved.

[ghost]

Closing in favor of #4498