Add new notification icon. Fix 2.3 notification compatibility.
Fix the test compilation failure for converters.
Refactory CardListActivity to show card learning status.

Major refactory CardListActivity with class name change and
menu system to use new recommended PopupMenu.
Color code cards for learning status.
Add menu for quick learning action (Not implemented yet).
Add mark as learned/forgotten function to card list activity.
Refactory "skip" to "Mark as learned forever".
Fix a few warnings. Add Search view in CardListActivity (Not implemented
yet)
Implement the search function in CardListActivity.
Add ability to show/hide answer in CardListActivity.
Use loader instead of AsyncTask to load Card list in CardListActivity.
Fix the learning data reset does not initialize the date objects.