Configurable, stateful organization

@sandersonet, see the README also, but this allows us to configure a somewhat stateful organization, which will be used as the basis for all documents/events. So, the following is possible:

```ruby
organization = Aptible::Auth::Organization.all(token: token).first
Aptible::Gridiron.configuration.organization = organization

gridiron = Aptible::Gridiron::Agent.new(token: token)
criterion = gridiron.protocols.first.procedures.first.criteria.first
criterion.name
# => "Policy Manual"
document = criterion.create_document!(
  print_version: 'http://knowyourmeme.com/photos/11296-success'
)
document.expires_at
# => 2015-07-08 00:10:31 UTC
```