Support for alternative to NIST elliptic curves

Support for alternative to NIST elliptic curves: There is a lot of hype around the curves defined by NIST and although there are many exaggerations, having alternatives is a good thing. New curves will be added as long as they are standardized and are supported by the nettle library.

Related drafts/RFCs:

• Curve25519: https://tools.ietf.org/html/draft-ietf-tls-curve25519-01
• Brainpool curves: http://tools.ietf.org/html/rfc7027