Now gitignore the Git-branch/hash-named ISO files.

- e.g. `frost-init-20141228.a0b05a0191707a75745f626e6ca61d4a4fffb983.iso`
  will be ignored, since there's no point to committing this binary file
  into a repository
- We were already preventing the default `init.iso` file from being versioned
- If you want to override this behaviour and store an ISO (not advised,
  and totally unnecessary), just name it something that doesn't follow
  that naming pattern. For instance, `frost-init.iso`, `frost-init-20141228.iso`,
  `frost-20141228.iso` will all fall outside of the scope of the gitignore pattern.