Merge branch '402-blog-client-implementation' into 'master'

Blog Client with Factories and Validator

This implements a simple initial blog client that covers the basic blog actions, but no deletion/removal of blogs, yet.

Classes for Blogs and Blog Post Headers have been introduced along with the associated factories.

A `BlogPostValidator` has been added that validates incoming blog posts.

Closes #402
Closes #404

See merge request !212