Add icons for Play Again and Info

Users had trouble finding and understanding the Play Again button.
New large buttons are planned, using both text and icons.  The Info
button will be completely new.

* Icons added from app-bits.com
* Readme credits app-bits.com according to license.
Use UIButtons for Instructions and Play Again

Needed larger buttons with icons.

* playAgainButton is now a UIButton instead of a UIBarButtonItem
* New Instructions button (currently no function, planned for a popover).
* buttons have images