Add hooks for post-specific data visualizations

[ginatrapani]

Right now you can add menu items/views on the dashboard, but now the post detail page.

Need to add hooks so you can add a view (like a list or visualization) of responses to a given post. For example, the Google Maps view of replies should be abstracted out to a plugin. A "Wordle" word map of replies is another visualization plugin that we want to implement for a set of responses.

Steps:

• Add _lib/model/interface.PostDetailPlugin.php that defines a getPostDetailMenu() method (like the interface.DashboardPlugin does now)
• Have plugins implement the PostDetailPlugin interface and that method
• The PostController should add the menu to the view and render it appropriately

[ginatrapani]

We should also add these same hooks to a specific user page. Then, developers could define views for three things: groups of posts (dashboard), an individual post and its related posts (RTs & replies), and a given network user.

[ginatrapani]

Done for post page: https://github.com/ginatrapani/ThinkUp/commit/ed2fb07bf1a3bea114f4a89397f8c9b0ae2f2a30