Add logging

[fsackur]

I needed more info on #4 . This doesn't shed any light in itself, but it's a needed feature.

Mostly fulfills #3

Drawback: it clobbers Write-Verbose, Write-Debug etc. until async part is complete and the logger is removed from scope.

Mitigation: logger writes to console using $PSStyle or $Host.PrivateData to format. Should be visually indistinguishable.