[with patch] add cremona's modular symbols program g0n to sage and add wrapper supprot

[was]

A first solid version of this is now done, due to work of Ralf PW, John Cremona, and myself.

A patch bundle that applies cleanly and the corresponding spkg are both available here:

 http://sage.math.washington.edu/home/was/cremona/

The wrapper code now has 100% doctest coverage, is structured correctly, and provides some useful functionality.

[mabshoff]

Merged in 2.8.13.alpha0.