Add loads of documentation
Change header in readme
Better explain what seen_through means
Clarify relationship between interest and subscription
Add comments to subscriptions helpers
Various documentation tweaks based on feedback:

- fix docstring for Interest#in
- remove XXX near Interest#create_subscriptions
- add more docs to SeenItem class
- add more descriptive @todo for SeenItem#subscription_id
- remove SeenItem#json_view
- add more descriptive @todo for Subscription#query
- replace docstring with @see for Subscription#search
- made some # internal comments into # @private
Correct errors in the description of the Interest class
Update a few option docstrings in the subscriptions manager
Merge pull request #450 from jpmckinney/docs

More documentation