Add throttling options

Suggestion is to adopt a [Rack Attack](https://github.com/kickstarter/rack-attack) configuration that allows to control request limiting per `Apicasso::Key`.

This project was designed to have this feature implemented based on JSON rule present on the `.request_limit` field on all `Apicasso::Key` object. The suggested format is:

```
{
limit: # AN ALL AROUND REQUEST LIMITING,
period: # AN ALL AROUND PERIOD FOR REQUEST THROTTLING,
<% paths.each do |path| %>
<%= path %>: {
limit: # A REQUEST LIMITING FOR A SPECIFIC PATH,
period: # A PERIOD FOR REQUEST THROTTLING FOR A SPECIFIC PATH
}
<% end %>
}
```

Please note this code example is just for the sake of explanation. This would make APIcasso have global and per-path request limiting, all based on which key is being used for the request. A feature like this would enable for API monetizing based on request quota selling

まず Apicasso::Key とリクエスト処理のエントリーポイントを見つけ、次に Rack Attack が .request_limit 設定をどのように利用できるかを確認します。キーの制限値と期間に従ってリクエストがグローバルおよびパス単位で制限され、提案された設定が一貫して表現されていれば完了です。

索引モデルが issue の本文から書いたものです。