[TASK][!!!] Remove old domain models

This is a first steps towards a node-based blog
[TASK] Adjust Readme and make Swiftmailer optional
[WIP][TASK] Adjust to new node creation

This incorporates several changes provided by Bastian Waidelich.

Known issue: old blog post folder structure and new post generation
structure don't match anymore.
[FEATURE] Protect PostController->createAction()
[TASK] Adjust creation of new posts to old YYYY/MM/DD scheme

This moves the creation of new blog posts from the command controller
to the Post action controller and emulates the current path scheme of
my website (…blog/posts/2013/02/26/my-blog-post.html).

It's not meant to be the final solution but is necessary for creating
new content on my site at the moment.
[TASK] Lower dependency on Flow version
[BUGFIX] Fix notifications to work without Swiftmailer installed