Improvements to library configuration.

Introduced a `Orchestrate::Configuration` object for configuring the library. Instead of providing a JSON file to Wrapper you can configure the library as such:

```ruby
Orchestrate.configure do |config|
  config.api_key = ...
end
```

Or for a specific client, you can pass a configuration object to it that will override `Orchestrate.config`:

```ruby
config = Orchestrate::Configuration.new(api_key: "foo")
client = Orchestrate::API::Wrapper.new(config)
```

Testing with a specific key can now be performed by running `TEST_API_KEY=the-key bundle exec rake test`.