Adds versioning for out-of-process storage driver

The registry currently only accepts storage driver versions with the
same major version and an equal or lower minor version as its own
current storage driver api version, but this may be changed in the
future if we decide to implement specific version cross-compatibility.
Initial open-design proposal
Expects storage driver executables to be of the form registry-storage-name

Moves main packages to sub-packages of the individual storage drivers
Merge pull request #685 from BrianBland/storagedriver

Expects storage driver executables to be of the form registry-storage-<name>
Renames ResumeWritePosition to CurrentSize in storage driver api
Merge pull request #699 from BrianBland/storagedriver-api-rename

Renames ResumeWritePosition to CurrentSize in storage driver api
Merge pull request #686 from BrianBland/storagedriver-versioning

Adds versioning for out-of-process storage driver
Initial V2 API Router Implementation

This commit includes the initial API router, based on gorilla mux and a test
suite ensuring the expected variables are extracted. Currently unexported, the
structure here will likely change as this definition will be shared with the
API client.
Merge pull request #701 from stevvooe/initial-api-routes

Initial V2 API Router Implementation
Initial open-design proposal
Merge pull request #642 from docker/init

Initial open-design proposal