Auto renewable support

This adds support for auto-renewable subscription receipt verification, which requires the iTunes Shared Secret to be provided.

### Salient points:

If you need this feature, you can set your shared secret in an initializer (most likely from an ENV variable)

```ruby
Venice.shared_secret = ENV['ITUNES_SHARED_SECRET']
```

That's all you need, subsequent requests using `Venice::Client` will send a `password` param with each request.

The `iap` binary is also updated to support an option to provide the requisite shared secret:

```ruby
iap verify receipt.txt --secret mysharedsecretdonttellanyoneplzkthx
```

There are tests for the client, but an integration test would require a real shared secret, which we obviously shouldn't provide.