Follow best practises around data models

[ashsmith]

We should follow best practises set out by Magento by using data models to access our blog posts.

I'm going to implement the following

• Ashsmith\Blog\Api\Data\PostInterface (this already exists)
• Ashsmith\Blog\Api\PostRepositoryInterface
• Ashsmith\Blog\Model\PostRepository
• Ashsmith\Blog\Model\Data\Post

Then modify Ashsmith\Blog\Model\Post so that it no longer uses the interface, and has the method getDataModel.

PostRepositoryInterface should define the following methods:

• getById($postId)
• getList(\Magento\Framework\Api\SearchCriteriaInterface $searchCriteria)
• delete(\Ashsmith\Blog\Api\Data\PostInterface $post)
• deleteById($postId)

Any reference to loading the collections, or the original Post model directly will be switched out for the PostRepository.