Compute M2 for server authenticity check
Merge pull request #1 from chiiph/computeM2

Compute M2 for server authenticity check
Add constant time equal
Implement RFC2945-compatible M1 calculation.

This breaks compatibility with Mozilla's PiCl, which uses a much more
trivial calculation.  Shame there isn't a nice interface to slot your
own M1 calculation function in.
Bumped bignum version to 0.7.0