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.
The current intention is that each driver will have a main() method that handles (json) serialized parameters when run out-of-process over ipc. If you reference [contrib/golang_impl_ng/main/driver/filesystem/filesystem.go](https://github.com/BrianBland/docker-registry/blob/8ef5c073f118709f215af1a12021959cd0fa9b1c/contrib/golang_impl_ng/main/driver/filesystem/filesystem.go), you can see that we're pulling the RootDirectory parameter out of the first command line argument, which is a json blob of all required configuration params.