Start on a split view to use on iPad.
Create a protocol for filtering tasks on an object.

Prepare to use the protocol to filter tasks in the task list based
on selections in the filter table view.
Show contexts and projets, and allow filtering on them.
Used a grouped table view for the sidebar on iPad.
Fix initial view setup on iPhone.
Use the checkmark accessory type to indicate selected contexts/projects for filtering.
Increase the robustness of filtering tasks.
Update license blocks.

Add license blocks to FilterViewController.h/m and update year range
in other files.
Rename TaskFilterable->TaskFilterDelegate
Use private properties rather than ivars in the app delegate.
Use private properties rather than ivars in todo_txt_touch_iosVC.