* README.rst: Updated minimum libgit version number to 0.18

* doc/cl-git.texinfo: Updated minimum version number to 0.18

* doc/index.rst: Fixed typo

* doc/repositories.rst: Added 3 new methods

* src/api.lisp (git-path, git-workdir): New methods

* src/references.lisp (git-target): Return object instead of oid

* src/repository.lisp (git-head, git-head-detached, git-path, git-workdir):  new functions.

* src/tag.lisp (git-list): New method for tags
             (git-tagger): Fixed bug found by Russell