Add SASL EXTERNAL mechanism

Add SASL EXTERNAL mechanism

Support for EXTERNAL mechanism itself is already in Smack, but aSmack uses a custom (internal) implementation of SASL, stripped down of the EXTERNAL mechanism. I retrieved it from the original openldap library (jldap) and modified it to import some stuff from Apache Harmony (like someone did in DigestMD5SaslClient).

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>