Initial implementation of registry LayerService

This change contains the initial implementation of the LayerService to power
layer push and pulls on the storagedriver. The interfaces presented in this
package will be used by the http application to drive most features around
efficient pulls and resumable pushes.

The file storage/layer.go defines the interface interactions. LayerService is
the root type and supports methods to access Layer and LayerUpload objects.
Pull operations are supported with LayerService.Fetch and push operations are
supported with LayerService.Upload and LayerService.Resume. Reads and writes of
layers are split between Layer and LayerUpload, respectively.

LayerService is implemented internally with the layerStore object, which takes
a storagedriver.StorageDriver and a pathMapper instance.

LayerUploadState is currently exported and will likely continue to be as the
interaction between it and layerUploadStore are better understood. Likely, the
layerUploadStore lifecycle and implementation will be deferred to the
application.

Image pushes pulls will be implemented in a similar manner without the
discrete, persistent upload.

Much of this change is in place to get something running and working. Caveats
of this change include the following:

1. Layer upload state storage is implemented on the local filesystem, separate
   from the storage driver. This must be replaced with using the proper backend
   and other state storage. This can be removed when we implement resumable
   hashing and tarsum calculations to avoid backend roundtrips.
2. Error handling is rather bespoke at this time. The http API implementation
   should really dictate the error return structure for the future, so we
   intend to refactor this heavily to support these errors. We'd also like to
   collect production data to understand how failures happen in the system as
   a while before moving to a particular edict around error handling.
3. The layerUploadStore, which manages layer upload storage and state is not
   currently exported. This will likely end up being split, with the file
   management portion being pointed at the storagedriver and the state storage
   elsewhere.
4. Access Control provisions are nearly completely missing from this change.
   There are details around how layerindex lookup works that are related with
   access controls. As the auth portions of the new API take shape, these
   provisions will become more clear.

Please see TODOs for details and individual recommendations.
Adds ability to unwrap ipc errors into their original type

This only works for a specific whitelist of error types, which is
currently all errors in the storagedriver package.

Also improves storagedriver tests to enforce proper error types are
returned
Merge pull request #741 from stevvooe/layer-service

Initial implementation of registry LayerService
Merge pull request #749 from BrianBland/storagedriver-ipc-errors

Adds ability to unwrap ipc errors into their original type
Update routes to reflect specification changes

To bring the implementation inline with the specification, the names and
structure of the API routes have been updated.

The overloaded term "image" has been replaced with the term "manifest", which
may also be known as "image manifest".  The desire for the layer storage to be
more of a general blob storage is reflected in moving from "layer" api prefixes
to "blob". The "tarsum" path parameter has been replaced by a more general
"digest" parameter and is no longer required to start uploads. Another set of
changes will come along to support this change at the storage service layer.
Merge pull request #751 from stevvooe/update-routes

Update routes to reflect specification changes
Move Digest type into discrete package

The Digest type will be fairly central for blob and layer management. The type
presented in this package provides a number of core features that should enable
reliable use within the registry. This commit will be followed by others that
convert the storage layer and webapp to use this type as the primary layer/blob
CAS identifier.
Move storage package to use Digest type

Mostly, we've made superficial changes to the storage package to start using
the Digest type. Many of the exported interface methods have been changed to
reflect this in addition to changes in the way layer uploads will be initiated.

Further work here is necessary but will come with a separate PR.
Various adjustments to digest package for govet/golint
Include testutil package needed for tar-based tests
Merge pull request #755 from stevvooe/digest-type

Move Digest type into discrete package
Updates client to newer routes and changes "layer" to "blob"