Adds initial volumes package; Supports host-dirs

Adds the framework for external volume mounts.

Currently supports bare host directory mounts.

Modifies the API to support host directory mounts from Volumes
instead of VolumeMounts.
Adds EmptyDirectory volume struct

Adds EmptyDirectory to volumes. This represents a directory
on the host, given to a pod that should not persist beyond.

The current draft does not cleanup after itself.
API modified to use source; now supports EmptyDirectory

API is now modified to use a Source struct to handle multiple volumes.

Two volume types are supported now, HostDirectory and EmptyDirectory.
Merge pull request #452 from Sarsate/extvol-hostdir

Initial framework for external volumes.