Entry model

www.agileplannerapp.com/boards/173808/cards/7897

An Entry stores the content_id, title, format and base_path
of a piece of content being edited in Publishing applications.

Ignoring the `state` attribute for now, because we need better
understanding of what's expected from that.

Making a couple of changes which remove warnings from test
output and ignore coverage reports in git.

Also, adding shoulda-matcher for consise validation specs.
Submit and fetch entries

www.agileplannerapp.com/boards/173808/cards/7897

the publishing applications use a content_id to
submit and Entry for a piece of content. for now
we need to retrive all entries for a particular
format.
Merge pull request #4 from alphagov/entry-crud

Entry resource CRUD