(#411) added logout parameter and forced write-host to honor that param

fix for issue #411 discussed at  https://groups.google.com/forum/#!topic/chocolatey/yCV3J6BuvpY

Added a new parameter, logout, which is true be default but now you can turn Write-Host off by setting it to true.
Tried to add tests for this but since the actual issue is Write-Host it gets hard to test for WH not being called, thus turning it off, and then Pester wont output anything. 
As I said I actually wrote some tests but they didn't feel right. 
Maybe I'm wrong here.
Tested locally with DSC (desired state configuration) and it work great.

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