Basic support for Pageant as SSH agent

On PowerShell launch, check to see if GIT_SSH exists and is set to
plink. If so, check to see if pageant is running and return if it is.
If pageant is not running, attempt to start it and load all .ppk
files stored in ~/.ssh

Requires pageant to be on path for it to be launched.
Pageant support for Add-SshKey()

Also changes Start-SshAgent() to use Add-SshKey() for both agents.
Add Pageant support to Get-SshAgent()

Also changes gets some information in a more intelligent manner.
Refactor Start-SshAgent code

Make paths for Pageant and ssh-agent more alike.
Add warning to Add-SshKey if Pageant not found
Switch to using -imatch, fix whitespace
Prevent a possible error in Stop-SshAgent
StrictMode support

Change to using Select -ExpandProperty to access properties.
Merge pull request #133 from theaquamarine/pageant

Add support for Pageant as SSH agent