[Closes #678] List Portlet

List portlets find and return content blocks. The following can be configured:

* Find all of a given content type and display as a list.
* Can limit results
* Can order results (uses AJAX)
* Can sort/reverse sort results
* Can chose list or table views
* Provide a way to custom the view for a given list portlet via creating an application specific version.
* Only works for html.erb files. Format is: app/views/portlets/list/#{name_of_portlet}/_#{view_as}.html.erb
* Name links if content is addressable.

Other Fixes:

* Fix portlet generator so it generates SimpleForm fields.
* New portlets have a prompt for developers to add a suitable description.