Only run Pageant during Ssh-AddKey if keys found

Fixes #143
Merge pull request #144 from theaquamarine/norun

Only run Pageant during Ssh-AddKey if keys found
Updated readme with installing via PsGet
Merge pull request #149 from vorou/patch-1

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.
Merge pull request #155 from pxc/master

Cope with multiple Pageant installs (one-line change)
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
Merge pull request #156 from ShadowWolf/provider-issue

sqlps's provider causes posh-git to throw errors