Add support for embedding elliptic curve public keys in KeyValue element

It seems Santuario does not currently support embedding elliptic curve public keys in the KeyValue element of an XML signature. See, for example:

http://svn.apache.org/viewvc/santuario/xml-security-java/trunk/src/main/java/org/apache/xml/security/keys/content/KeyValue.java?revision=1450197&view=markup

This ticket is to request that this support be added. As EC certs gain in popularity, this seems like something that would be nice to have.