add background color for even amccode (Feature #99)

Proposed to make it optional.

i made it.

this the code:

 \AMCboxDimensions{shape=oval,size=2.2ex,down=1ex}
 \begin{tikzpicture}
\begin{pgfonlayer}{background}
\node [fill=black!00] {
\AMCcodeHspace=0.48em\AMCcodeVspace=0.48em\AMCcode{StudentNum}{10}
}; % enough to add two diagonal nodes
\end{pgfonlayer}
\draw[step=0.53cm] (-2.66,-2.651) grid (2.65,3.18);
\end{tikzpicture}

print and shaded student numbers by calc : heeeerhttp://project.auto-multiple-choice.net/boards/2/topics/621

if you don't have 10 numbers like "4326" or "43" or "686743" ..
you can complete the 10 numbers by put your countries number from :
http://en.wikipedia.org/wiki/List_of_GS1_country_codes
or
http://www.makebarcode.com/specs/ean_cc.html

example1 :
if you are using 4 numbers for the students numbers and you are from saudi arabia ?
EAN number for saudi arabia is = 628
so you can write students numbers like this..
6280004001
6280004002
6280004003
....

example2 :
if you are using 6 numbers for the students numbers and you are from Finland ?
EAN number for Finland is = 64
so you can write students numbers like this..
6400600001
6400600002
6400600003
....