Checkout default branch if directory exists on install

```bat
mk help install

  Usage: index-install [options]

  Options:

    -h, --help                   output usage information
    -n, --noprompt               Automatically clone all repositories without prompting
    -l, --links                  Create npm & tsd symbolic links after installation
    -i, --npmi                   Execute npm install on all repositories
    -a, --all                    Run with all options [-n,-l,-i]
    -d, --default                Checkout default branch if the repository is already cloned
    -e, --exclude <project;...>  Project name to exclude, semicolon seperated
```

<!---
@huboard:{"order":1.1920928955078125e-07,"milestone_order":6,"custom_state":"ready"}
-->