Support scrypt into SRP6a in order to avoid weak password storage

[fpietrosanti]

The default key material storage of SRP is considered to be weak (see Solar Designer here http://lists.randombit.net/pipermail/cryptography/2015-March/007121.html) .

For this reason we suggest, following discussion on randombit cryptography mailing list, to integrate scrypt as an additional custom hashing method to SRP as described by Alfonso De Gregorio (see http://lists.randombit.net/pipermail/cryptography/2015-March/007123.html) .

It's suggest to use the the following scrypt library This ticket ishttps://github.com/dchest/scrypt-async-js (by Solar designer, see http://lists.randombit.net/pipermail/cryptography/2015-March/007117.html).

By improving SRP authentication, bundling it with scrypt KDF, the authentication protocol would also feature strong password storage server side.