Move ~/.gitconfig from copy/ to link/ . Use include for user.name

* Move `user.name` and `user.email` to `~/.gitconfig.local`
* `[include]` the file `~/.gitconfig.local` from `~/.gitconfig`
* Move `~/.gitconfig` from `copy` to `link`

As of git 1.7.10+, it's possible to specify the following syntax in `~/.gitconfig`:
```
[include]
     path = /path/to/file
```
The benefit of having `.gitconfig` in `link/` instead of `copy/` is that updates to git aliases can be kept up to date by just running git pull on the dotfiles repo.