Custom git binary support

Provide support for specifying a custom `git` executable. Since we need to ensure that our omnibussed `git` is executed without mucking around with my PATH, there is now a `@@git_binary` variable in `Git::Lib` which can be set via `Git::Lib.git_binary = '/path/to/git'`.
I would avoid the usage of the class variable here and use the method #get_binary method instead.