Crypto.ECC.Curve: Added some more PKCS APIs.

Adds sufficient pkcs_*() functions to generate selfsigned certificates.
Crypto.Sign: Added base class for signature algorithms.

Crypto.Sign is to contain the APIs common to Crypto.RSA, Crypto.DSA
and Crypto.ECC.Curve.ECDSA.
Crypto.ECC.Curve.ECDSA: Added some helper functions.
Standards.ASN1.UTC: added a test

This test fails. I suspect the date is parsed incorrectly in local timezone
instead of UTC.
Standards.X509: Support ECDSA certificates.

Standards.X509.decode_certificate() now knows about ECDSA certificates.
Crypto.ECC.Curve: Added pkcs_named_curve_id().

This is to make it easier to identify curves given the ASN.1
identifier.
Crypto.ECC.Curve.ECDSA: Improved support for ANSI x9.62.

set_public_key() now accepts an ANSI x9.62 encoded public key.