Adds prototype NG driver apis

Includes an IPC system for out-of-process drivers so users won't have to recompile the registry to bring their own drivers. Also, drivers can potentially be written in any language with a functioning [libchan](https://github.com/docker/libchan) implementation.
On Mon, Oct 13, 2014 at 04:43:12PM -0700, Matthew Fisher wrote:
> In terms of previous filesystem driver configuration, there was a
> lot more than just the root directory. For example, in the S3
> driver, you had to set up your access key/secret key to connect to
> the S3 bucket. For the swift driver, you needed to specify your
> tenant name, region name, your keystone API key, etc. How will this
> fit in with this implementation?

Detached libchan storage drivers can just read their own config files
or CLI arguments or whatever when they spin up.  For same-process
drivers, I'm fine with either the current “just pass through the whole
config hash structure” or a similar approach using environment
variables.