Interface breaking changes.

Remove the entity class.
Make 'discover' a standalone function.
Rename the app class to client.
Massively simplify what data is stored with the client class.
Add a requirements file back in.
Make significant internal changes.

Discover now returns the meta post, not the meta post envelope.
Create a PostListPage class.
Automatically add app_id to credentials.
Fix bug in the delete post method.
Merge pull request #5 from seagreen/morelikethegoclient

Make breaking changes to simplify the interface.