Use 4096-bit DH prime, equivalent to AES-128

We're using DH over a finite field modulo a prime, not ECDH. To have the same strength as a 150-bit symmetric cipher key, against the best attack techniques 2004 had to offer, we'd need about a 4575-bit DH modulus.[1] AES-128 is about as hard to break as a 3200-bit DH modulus given the best techniques of 2001.[2]

[1] https://tools.ietf.org/html/rfc3766 (see table in section 5)
[2] http://tools.ietf.org/html/rfc3526
2048-bit DH modulus provides 100-bits of security

According to the table in section 5 of https://tools.ietf.org/html/rfc3766 a 2048-bit safe prime provides over 100 bits of security.  On my laptop, a 2048-bit safe prime takes about 2 minutes to generate, but a 4096-bit safe prime takes tens of minutes to generate and is still going.
Merge pull request #1 from kmag/patch-1

Use 2048-bit DH prime