Replaced std::shared_ptr<Monomial> by Monomial::Arg
Guarded operators with Coeff, Monomial::Arg by EnableIf to prevent ambiguity (see http://eigen.tuxfamily.org/bz/show_bug.cgi?id=921)
Merge branch 'master' of https://srv-i2.informatik.rwth-aachen.de:8443/git/carl