Add initial draft of SRP spec

With this pull request, I'd like to open up a discussion about this draft SRP spec for zeroMQ.
It is by no means finished, but I'd like to gather feedback while I finish it and this is the best way of doing that.

A few of the things open for discussion, as well as obviously this whole document:

* What hashing algorithm do we want to use ? The obvious choices here seem blake and sha-2. With the recent fuss around the NSA & NIST, perhaps blake is a better choice ?
* The random prime to pick. The size is the main important difference here, there are primes available ranging from 1024 bits to 8192 bits.
* The symmetric encryption algorithm to use after the session key has been negotiated.