Implement MD5-sess

[catwell]

This implementation only supports the MD5 algorithm. We could implement MD5-sess as well.

However MD5-sess is not very useful and is stateful (HA1 must not be recalculated after the 1st request, although many people misread the spec and do it anyway, see this bug...). I do not think there is any value in doing this, at least not as long as we do not have nonce reuse (see #3).