Check validity of package file signatures, if they are signed

During the installation phase of a package, we should check to see if the binaries to be 'installed', and the scripts being used for that installation, are signed. If they are signed then we should check their signatures to ensure it is valid and stop installation if they are not.

One can do this now in the automation scripts (it's just PowerShell), but adding a facility to easily add the ability to verify the authenticode scripts would be awesome!

We may be able to look at binaries and build some of this into Chocolatey itself. To do this within Chocolatey itself, we may need to expand the scope so it does differencing against things like Program Files to see things installed.

Some possible resources:
- http://stackoverflow.com/q/7622732/18475
- http://stackoverflow.com/q/3281057/18475
- http://stackoverflow.com/q/7780324/18475
- http://stackoverflow.com/q/31879886/18475
- http://stackoverflow.com/q/6596327/18475
- http://certhelp.ksoftware.net/support/solutions/articles/50357-how-do-i-verify-a-digital-signature-in-c-
- http://geekswithblogs.net/robp/archive/2007/05/04/112250.aspx
- http://security.stackexchange.com/q/26485
- https://support.microsoft.com/en-us/kb/323809