Remove the old Tar module

We should use ocaml-tar instead, which is based on this but with added goodness:
* support for efficient marshalling/unmarshalling via Cstruct
* unit tests automatically run on every pull request from travis
* available in opam

Note note note: this can't be merged until the following are ready:
1. the .spec file for ocaml-tar is added so planex can build it
2. a small tweak to xapi to use the slightly differently-named modules