adds post model
view posts via api at posts#show
create posts via api at posts#create

Also refactors iso8601 validation into dedicated validator.
update posts via posts#update

 - editor is the currently logged user making the request
makes posts taggable
updates autoload paths to include `/app/validators`
adds slugs for posts

 - defaults to first 10 words of post title
 - falls back to first 10 words of content
Merge pull request #3 from openmcac/posts

Posts