Auto API generation

[jfahrer]

Summary

This PR implements a basic version of a HttpController for automatic JSON API generation.

What's new

Volt::RestfullBaseController

For server side controllers you can now inherit from Volt::RestfullBaseController. You can either pass in the model as a param or set it via model :name in the controller. The later overwrites the param.
The following helpers are available:

• model The name of the model as symbol
• collection The collection on the store repo
• collection_name The name of the collection
• resource The instance of the model based on the given id.
• resource_params The params for the resource. Params are expected to be nested with the model name as key. For example todo: { name: 'the_name_for_the_todo' }

The controller also sets up your resource based on the action:

• create A new instance that can be appended to the collection. The resource_params have already been applied to the instance
• update A buffered version of the model. You can update it via resource.update(resource_params)
• show A resolved version of the model (using sync)
• delete A resolved version of the model (using sync)
• index No resource will be setup

Volt::SimpleJsonController

A server side controller that inherits from Volt::SimpleJsonController automatically generates a simple JSON API for your models. You can either set the model via model :name in the controller or pass in the model via the params. You can do so in your routes file:

rest '/api/issues', component: 'api', controller: 'api', model: 'issue'
rest '/api/todos', component: 'api', controller: 'api', model: 'todo'
get '/api/cats', component: 'api', controller: 'api', action: index, model: 'cats'

Volt::SimpleJsonController implements the index, show, create, update and destroy actions for you. For updating and creation you need set the Content-Type of the request to 'aplication/json'. The JSON itself has to be prefixed with the model name.
For example: { "todo": { "name": "the_name" }}

Volt::SimpleJsonController inherits from Volt::RestfullBaseController. So you have the same helpers available.

What's missing

• Authentication
• Pagination: My idea here was to implement the pagination in the Volt::RestfullBaseController. per_page (and a max_per_page) could either be set via the params or directly on the controller.
• Make the Volt::SimpleJsonController more generic. So you could set the format via the URL or content type and call the corresponding renderer.
• Volt::HttpController does not have access to the routes. This is necessary to return the new location for a created instance. The corresponding code and test are in the code.
• Error handling. Especially for validations and permissions
• Generators for the new controller types
• Documentation