Add Project#default_root

On Windows-based systems, this value defaults to +C:+. On non-Windows systems, this value defaults to /opt. In this way, the user can transparently define an install_dir without the need to use branching logic on different platforms:

```ruby
install_dir "#{default_root}/#{name}"
```

Fixes #308 

/cc @jmink @opscode/release-engineers