Guess agent location if not on path

Implements #114 for ssh-agent and pageant
Maybe save off the result of `Get-Command git` so you don't have to look it up twice? You could also pipe that directly into `Get-Item` to save a bit of nesting...

```powershell
$gitItem = Get-Command git -Erroraction SilentlyContinue | Get-Item
if ($gitItem -eq $null) { ...; return }
$sshLocation = Join-Path $gitItem.Directory.Parent.FullName bin
```