Very simple SASL mechanisms implementation

Before this I've already done two gsasl bindings for python:
  * one was implemented using Cython;
  * second was implemented using vala.

The main problem with gsasl, is what it's callback mechanism was made
  without possibility to pass user data, so it is spiky to trying to
  make binding for OOP language like Python.

As I very mutch do not like non-OOP programming, finally I has dicided
  to write very simple SASL mechanism at least for PLAIN method, for
  not return to gsasl problems ever again.

I've written this package looking on gsasl, so it has some
  similarities, but I've also tryied to make it as simplier as
  possible, as I'm in horry and need to get My new mail server sofware
  working as soon as possible.

---

=====
Usage
=====

This SASL mechanism architecture shold be simple

1. import wayround_i2p.sasl.sasl

2. use init_mech(mech_name, mode, callback=None) function
   to create SASLSession instance.

   NOTE: at time of this writting only supported method(mechanism) is
   	 PLAIN and it's 'server' and 'client' side.

   example: init_mech('PLAIN', 'server')

   NOTE: callback is MAY only be needed in client mode, possibly for
         organizing dialog boxes. so this is like in gsasl.

   callback, if supplied, must be callable with one parameter - the
   name of field which server wishes client to return. The return
   value of callback will be then assigned to same name property in
   client's object '.properties' dict property

3. Use step() or step64() method of SASLSession instance to pass data
   to it's working mechanism.

   Those methods allways return two values:
      1. code - indicating error, success or asking for more data;

      	        At time of this writting, valid values considered to be:
		'ok', 'error', 'need_more'.

		gsasl supports more error codes indicating various
		   errors, depending on mechanism being used. and this
		   step() methods should also somehow specify errors.
		   possibly, the good way for farther development of
		   this methods (and particularly this package
		   mechanisms) is, in case of errors, return strings
		   starting with 'error' text and continuing with
		   '_some_spec' to indicate specific error.

      2. text - data wich should be sent to server or client
                counterpart. (In case of error, usually no any data
		should be sent farther, but read documentation
		on specifics of mechanism being used)

   NOTE: step64() is simply a wrap for step() method. step64() decodes
         it's input, passes decoding result to step(), after what
         encodes step() result and returning it's to caller.

=====================
Developing Mechanisms
=====================

To develop new mechanism - create new file with it's name in 'mechs'
directory. Write some code: take plain.py for example.

When Your new mechanism module is complete, register it in sasl.py, by
editing 'MECHANISMS' dict in sasl.py top.

The structure of mode class REQUIRED to have folloving things:
    1. def __init__(self, callback=None)
    2. def step(self, data)
    3. step() method MUST return 2 values:
       1: str
       2: bytes
    4. '.properties' dict property

Anythin other is on conscience of programmer.