Pinning

[RangerMauve]

As per #1081, this adds the dat pin command.

This also deprecates the dat registry commands.

Some things I'd like feedback on in addition to the code:

• Should I change any command names?
• Should the pinning service only keep the latest files instead of the full history?
• Was it a good idea to deprecate the registry stuff?

[joehand]

(much more to review)

Can you make the config file location in dat-pin configurable, then set it explicitly in the CLI. This will make it easier to change it/make it an option.

EDIT: oops, it is already configurable. Can we set it also in the CLI? That way we dont have to dig down the dependency tree later to figure it out =)