Trac #16840: Enforce that bistochastic matrices have nonnegative entries

Currently it is not checked that bistochastic matrices have nonnegative
integer entries. However, this is one of the conditions
[http://en.wikipedia.org/wiki/Doubly_stochastic_matrix] and the
Birkhoff–von Neumann theorem does not hold otherwise.
{{{
sage: m = matrix([[-1,2],[2,-1]])
sage: m.is_bistochastic()
True
}}}

URL: http://trac.sagemath.org/16840
Reported by: aschilling
Ticket author(s): Anne Schilling
Reviewer(s): Arvind Ayyer
Trac #15246: add xkcd function to sage

Sage already has an xgcd function but no xkcd function. We should really
change that.

URL: http://trac.sagemath.org/15246
Reported by: mderickx
Ticket author(s): Maarten Derickx, André Apitzsch
Reviewer(s): Travis Scrimshaw