Support handling host attributes

With this branch, serverspec can handle host attributes.

This pull request implements ``attr_set`` and ``attr`` methods.

You can set attributes like this.(This may be set in spec_helper.rb)

```ruby
attr_set :server_id => 100
```

And you can use this attributes in your spec like this.

```ruby
describe '/etc/my.cnf' do
  it { should contain "server-id = #{attr[:server_id]}" }
end

```

This pull request provides only methods for setting and getting attributes.You can use any external resources you like.