New functionality to add a shopping cart-like workflow.
Major changes:
 - Do not assume that seats array in session is all in one show.
 - Do not clear seats array unless the user requests it.
 - Seats array now indexed by a code containing showid and seatid.
 - New buttons for add to cart, clear cart, checkout, etc.
 - New form in confirm page to remove seats from cart.
 - New markup to box in the cart display.
Debug call to array_walk by including namespace.
Changing seats array index for consistency.