Fix powershell streams leaking to standard error

[mwrock]

Powershell has several streams. Unless, output is sent to the output stream, winrm interprets the output as error output. There are some module auto loading scenarios where powershell will send information out to the progress stream. This suppresses that stream all together by turning it off.

Also, the checksum script currently writes the hash using write-host. As of powershell v5, output sent using write-host directs some metadata to a new information stream and suffers the same problem as the progress stream where it causes winrm to leak to standard error. This simply replaces write-host with write-output which will be output to stdout and the console.

This fixes #18

[sneal]

Nice job, thanks!