Use secp521r1 curve instead of sect571k1

It's in every ssl libs
(http://en.wikipedia.org/wiki/Comparison_of_TLS_implementations)
and for openssl it's in every distribution
(for exemple fedora 20 only has secp521r1, secp384r1, prime256v1)

Signed-off-by: Etienne CHAMPETIER <etienne.champetier@free.fr>