add config class and configuration for default headers

Per basecrm/basecrm#17

1) Added a config class where you can specify default headers, which can get around the issue of the headers not being passed down when instantiating resources.

So now I can do:
``` ruby
ApiClient.configure do |config|
  config.default_headers = {"X-Pipejump-Auth"=> BASE_API_TOKEN, "X-Futuresimple-Token"=> BASE_API_TOKEN}
end
```
And not be forced into instantiating client sessions, which like I said is very useful for single organizations. Particularly when working with the api objects directly i.e.

``` ruby
BaseCrm::Lead.create({:last_name => "whatever"})
```