Run the animation timer on the main thread to fix interaction bug

Run the animation timer on the main thread.  We want to allow the user
to interact with the UI while this timer is running.  If we run it on
this thread, the timer will be halted while the user is touching the
screen (that's why the chart was disappearing in our collection view).
Merge pull request #20 from miketsprague/master

Fixed a bug that cancelled the animation timer and made the pie chart disappear whenever the user clicks on the animating chart.