Opening price is "N/A" before the market opens

Before the market opens, Yahoo returns the string "N/A" for the opening price,
which causes the float() operation to raise an exception.

Signed-off-by: Timur Tabi <timur@tabi.org>
Move the Quit menu to the bottom

Signed-off-by: Timur Tabi <timur@tabi.org>