Updated image v1 spec after more review

- Removed description of "Image" from the terminology section. The entire
  document is meant to serve this purpose.
- Updated the definition of "Image Filesystem Changeset".
- Clarified the level of randomness needed for generating image IDs.
- Updated the description of "Image Checksum".
- Added term descriptions for "Repository" and "Tag"
- Removed extraneous/implementation-specific fields from the Image JSON
  example file and field descriptions:
  - removed "container_config" and "docker_version" fields.
  - Added missing "author" field example and description.
- Removed extraneous/implementation-specific fields from the "config" struct
  example and description:
  - removed "Hostname", "Domainname", "Cpuset", "AttachStdin", "AttachStdout",
    "AttachStderr", "PortSpecs", "Tty", "OpenStdin", "StdinOnce", "Image",
    "NetworkDisabled", and "OnBuild".
- Updated example Image JSON config with better example values for "Env",
  "Cmd", "Volumes", "WorkingDir", "Entrypoint", "CpuShares", "Memory",
  "MemorySwap", and "User".
- Added notices that any fields not specified are to be considered as
  implementation specific and should be ignored my implementations which
  are unable to interpret them.
- Updated example of creating layer filesystem changesets to use less formal
  language.
- Listed more details in the section regarding extraction of a bundle of image
  layers into the root filesystem of a container.
- Updated the closing mention of Docker as an evolving implementation.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)