Possibility of making the filestore an external dependecy.

Right now making any part of the filestore an external dependency will be difficult, but not impossible. The major reason for this is because the filestore depends on other parts of IPFS. (In fact in order to avoid circular imports I had to break the filestore into several packages) Here are the packages and the deps within ipfs of the full filestore implementation for reference:
```
filestore:
github.com/ipfs/go-ipfs/filestore/pb
github.com/ipfs/go-ipfs/merkledag/pb
github.com/ipfs/go-ipfs/thirdparty/ds-help
github.com/ipfs/go-ipfs/unixfs/pb

filestore/pb:

filestore/support:
github.com/ipfs/go-ipfs/blocks
github.com/ipfs/go-ipfs/blocks/blockstore
github.com/ipfs/go-ipfs/filestore
github.com/ipfs/go-ipfs/merkledag
github.com/ipfs/go-ipfs/thirdparty/ds-help
github.com/ipfs/go-ipfs/thirdparty/posinfo
github.com/ipfs/go-ipfs/unixfs/pb

filestore/util:
github.com/ipfs/go-ipfs/blocks/blockstore
github.com/ipfs/go-ipfs/blocks/blockstore/util
github.com/ipfs/go-ipfs/commands
github.com/ipfs/go-ipfs/core
github.com/ipfs/go-ipfs/filestore
github.com/ipfs/go-ipfs/filestore/support
github.com/ipfs/go-ipfs/merkledag
github.com/ipfs/go-ipfs/pin
github.com/ipfs/go-ipfs/repo/fsrepo
github.com/ipfs/go-ipfs/thirdparty/ds-help
github.com/ipfs/go-ipfs/unixfs
```

The other major stumbling blocks are the handling of the add command that depends on implementation details of the adder, and the `filestore` sub-command.