Json Web Keys again

[Angus Robertson - Magenta Systems Ltd]

> There isn't a key specific format for Ed25519.
> You need to use i2d_PUBKEY() for that.

I used EVP_PKEY_get_raw_public_key which got added for these raw keys,
works fine for Ed25519.   

On the EVP_PKEY_get_raw_public_key.html page, it would help if it
mentioned that *len should be set to the passed buffer size on the
get_raw functions which fail if you pass zero. 

Angus