Allow specifying template and partial locations.

This change allows for more than one rendering of an activity (list.)

example:
```ruby
  # Large, interactive rendering
  = render_activities(@activities, :root => "/profile/activity", :layout => "/profile/activity/layout")

  # Small, terse rendering
  = render_activities(@activities, :root => "/sidebar/activity", :layout => "/profile/sidebar/layout")
```