Extended features for todo app

Add support for completion due dates and priority for items
Change the item in ListView to display additional attributes, stylize
the items
Tweak the style of the app, improve UI, play with adding images or
backgrounds
Persist the todo app in SQLite instead of using a text file
Use a DialogFragment instead of new Activity to support editing items
Also used DialogFragment for adding new todo item
Added ActionBar item to trigger adding of new item