EC private key support

[gugu]

There is a new elliptic curve private key notation, which is different to the average private key. jwt.io currently does not support this. jsonwebtoken module handles both of them correctly.

It is possible to convert EC public key to supported format manually using this command:

openssl pkcs8 -topk8 -nocrypt -in ec1.pem -out ec2.pem

but it would be greate to have it supported natively. It will solve issues like #606