ext/openssl/ossl_pkey_ec.c, Add method private? as alias of private_key?

When an ECC key is used to try and sign a pkcs7 message in the OpenSSL module the operation causes an exception as demonstrated here: https://gist.github.com/Jacob640/5239454

This patch rectifies the issue by creating a private? method for OpenSSL::PKey::EC that is an alias of OpenSSL::PKey::EC.private_key?

Also added is an improved warning when OpenSSL::PKey::EC.private_key= is used before a group has been set.