Fixes ipfs/go-ipfs#2155 by turning the hash path arguments into keys
and unpinning directly, rather than running a full core.Resolve on
them. This lets users fail fast when they try to remove pins that
they don't have locally.

Note that this will only work when the path is of the form <hash> or
/ipfs/<hash>. Given e.g. /ipfs/<hash>/foo, foo's key cannot be known
without first resolving <hash>, which may involve talking to the
network.

License: MIT
Signed-off-by: Stephen Whitmore <noffle@ipfs.io>