Prefer stronger algorithms in algorithm negotiation.

Prefer diffie-hellman-group14-sha1 (2048 bit) over
diffie-hellman-group1-sha1 (1024 bit).

Due to meet-in-the-middle attacks the effective key length of
three key 3DES is 112 bits. AES is stronger and faster then 3DES.

Prefer to delay the start of compression until after authentication
has completed. This avoids exposing compression code to attacks
from unauthenticated users.

(github pull request #9)