Feature/workdir for path

This patch provides a `workdir` method for character vectors, so that the git working directory can be identified for arbitrary paths e.g.

```
somefile='/path/to//my/repo/containing/somefile.txt'
repo=repository(workdir(somefile))
head(repo)
```
The implementation depends on libgit2's [`git_repository_discover`](http://libgit2.github.com/libgit2/#HEAD/group/repository/git_repository_discover) function. The patch could perhaps be improved by adding support to `git2r_repository_discover ` for the `ceiling_dirs` argument.