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.
Only run Pageant during Ssh-AddKey if keys found

Fixes #143
Updated readme with installing via PsGet
Cope with multiple Pageant installs

If Pageant is installed multiple times (e.g. inside both TortoiseGit and TortoiseHg) then just pick the first one.
LocalOrParentPath checks provider name

Resolves an issue for weird or obscure providers that do things like WMI
queries on directory checks. Primarily solves the SQLSERVER problem.

Resolves Issue #156
add LESS env var