3.1.2

CHANGES:

• Fix a wrong assumption in Tar_lwt_unix.run for Tar.Really_read _ that one
  tar block was always read. This meant that using Tar.Really_read with a
  size different from 512 would fail. (Reported by @jonahbeckford, review by @hannesm, @reynir, #153)
• Document better the actual behavior of Tar_unix.extract and
  Tar_lwt_unix.extract (@reynir, #155)