ssh: Add StripCRReader for gocrypto on windows

[bz2]

Fixes lp:1468752 by removing carriage returns from textual stdin
stream on windows when using gocrypto to communicate with remote
machine.

This is part one of making ssh from windows to a unix machine
more usable. Notably, this doesn't enable the passing of control
codes through to the remote shell, so for instance using a text
editor is still problematic.

QA steps

• Build juju using this version of utils for windows.
• Bootstrap with the resultant juju.exe
• juju switch controller
• juju.exe ssh 0
• On remote machine, ls ~
• See outout from ls and no other kipple.
• Run other commands as well, note vi somefile is still not usable.

[natefinch]

Personally, I'd make WrapStdin just check runtime.GOOS so that we don't need two files for single line functions, and it means you can test it more easily from any platform. However, I don't think it's a dealbreaker.

[natefinch]

Sorry, meant to make the one above an approve, but forgot to set it, and there seems to be no way to change that after the fact.

[bz2]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju-utils