Added constraint to print-items protocol

* src/protocol.lisp (print-items): adapted documentation string
  (format-print-items): adapted parameters, implementation and
  documentation string
* src/util.lisp: new file; contains utility functions used by the
  print-items system
* src/package.lisp (package print-items): added used packages
  alexandria, let-plus and iterate
* print-items.asd (system print-items): added system dependency on
  iterate; added file src/util.lisp
Use EOS instead of Lift for unit tests

* test/protocol.lisp: new file; contains initial unit smoke test
* test/package.lisp: new file; package definition for unit tests
* print-items.asd (system print-items-test): changed system dependency
  lift -> eos
Initial README draft in README.org

* README.org (Introduction): initial draft