Added git_index_read wrapper.
* src/branch.lisp: - Added new function git-is-head,
                   - removed non existent funciton git-branch-tracking, and git-tracking
		   - added doc string git-list for :branch
		   - added doc string git-lookup for :branch
		   - added new specialization for git-lookup for :branch

* src/index.lisp: - use git_index_add_bypath instead old incompatible git_index_add
                  - new function git_index_write_tree to
		    replace removed function create-tree-from-index

* src/oid.lisp: - lookup-oid uses now renamed method.

* src/package.lisp: - updated for new and renamed functions.

* src/references.lisp: - renamed git_reference_oid -> git_reference_target
                       - renamed git_reference_create_oid -> git_reference_create
		       - renamed git_reference_create_symbolic -> git_reference_symbolic_create
		       - updated git-create for renamed functions.
		       - new specialization for git-target

* src/remote.lisp: - fixed translate-from-foreign on fetchspec to properly return empty lists.
                   - small update for new cffi
                   - added doc string for git-connect, git-name, git-connected,
		                          git-pushspec, git-fetchspec
		   - renamed git-disconnected -> git-disconnect

* src/tree.lisp: - removed git_tree_create_fromindex because it does not exist anymore

* src/utils.lisp: - small change for new cffi in translate-from-foreign.

* tests/commit.lisp: - Updated tests for new names
* tests/common.lisp: - Updated for new names

* doc/branch.rst: - New documentation file
* doc/commits.rst, doc/index.rst, doc/references.rst, doc/remote.rst, doc/tree.rst:

  small documentation changes.