Feature/blog post images

added the model and controller for adding images to blog_post

if it's important i will make another commit to change the Picture model to Image. i would do that in a separate Pull Request because this PR is pretty big as it is

at the present moment adding images has been disabled but it will be implemented in the next PR which will also have Integration tests
I don't think this is correct. For one, has_many should reffer a plural (and `:blog_imageable` is singular)