Add xxhdpi application icon
Perform push operations synchronously.

Each call to push will increment a counter of pending push operations,
then launch a remote push in the background if one is not already running.
The background operation will clear the counter before starting.
After it completes, it will check the counter and relaunch itself if needed.
This will prevent race conditions that occur when multiple push operations
are in progress at the same time.

Push and pull operations will still not occur simultaneously, as always.
Re-enable sync indicator when pulling. Fixes #282

Fixed a couple of display issues with the taskage field on the task list.
When views are reused, it would sometimes show a date for tasks that had none.
Also, it would show dates even when the Date new tasks preference was not set
(if it had never been set).
Then, I re-enabled calling updateSyncUI when starting a pull. This was
commented out in 180e8bd because it supposedly messed up the current scroll position.
I couldn't reproduce that problem, but did notice the issues noted above. Regardless,
there is no reason to mess with the list when we just want to make the sync icon spin,
so I modified the method to take a boolean parameter. The list is redrawn only when the
parameter is true, and the only time we pass true to the method is after downloading a new
todo.txt file.
Merge pull request #305 from chuckbjones/272-race-condition-fix

Perform push operations synchronously, closes #272
Merge pull request #307 from chuckbjones/282-sync-icon-fix

Re-enable sync indicator when pulling. Closes #281
Merge pull request #299 from intrications/xxhdpi-icon

Add xxhdpi application icon
Fix issue #206
Remove Dropbox API key from AndroidManifest.xml
Closes #309
Restore db- prefix for Dropbox app key reference in manifest
Update Copyright notice to 2013