Trac #16271: Support the generic Steenrod algebra at the prime 2

For some applications one would like to work with the "odd primary
Steenrod algebra at the prime 2". The attached patch introduces a new
keyword "generic" that allows to do this:

{{{
  sage: EA=SteenrodAlgebra(p=2,generic=True) ; EA
  generic mod 2 Steenrod algebra, milnor basis
  sage: EA[8]
  Vector space spanned by (Q_0 Q_2, Q_0 Q_1 P(2), P(1,1), P(4)) over
Finite Field of size 2
  sage: SteenrodAlgebra(p=2,generic=True,basis='adem')[7]
  Vector space spanned by (beta P^2 P^1, beta P^3, P^2 P^1 beta, P^3
beta) over Finite Field of size 2
}}}

URL: http://trac.sagemath.org/16271
Reported by: cnassau
Ticket author(s): Christian Nassau
Reviewer(s): John Palmieri