New ciphers

This adds support for new ciphers introduced in openssh-6.4.

- Added host keys: `ssh-rsa-cert-v01@openssh.com ssh-rsa-cert-v00@openssh.com ssh-ed25519-cert-v01@openssh.com ssh-ed25519`
- Added HMACs: `hmac-sha2-512-etm@openssh.com hmac-sha2-256-etm@openssh.com umac-128-etm@openssh.com`
- Added Kex: `aes256-gcm@openssh.com aes128-gcm@openssh.com curve25519-sha256@libssh.org`
- Added private key support for `id_ed25519`

This addresses errors like the following: 

```
NotImplementedError: unsupported host_key algorithm: `ssh-ed25519-cert-v01@openssh.com'
```