[MERGE] Support for Putty SSH implementation

[Harald Meland]

[Alexander Belchenko]

> Dmitry Vasiliev пишет:
>> 
>> The attached patch adds support for Putty's plink SSH implementation.
>> Also slightly refactored SSH negotiation code and added some tests.
>
> What's improvements of using plink instead of paramiko?

I would guess that Pageant (putty's "ssh-agent" equivalent)
integration to be easier achievable (if not already present) when
using plink than with paramiko.
-- 
Harald