Releasing an x86 version of the CNG provider

Hello,

The current version of the CNG provider is targeting x64 only and while this works, it's a bit counter intuitive as the vast majority of online tutorials for signtool all refer to the x86 version.
As a result, one will get the dreaded "no private key is available" error message as described in Issue #17.

Further to this, Microsoft is providing various accompanying tools for use with signtool in the form of Subject Interface packages that allow signing files beyond the usual binary or powershell script. One of the most popular is for [Microsoft Office](https://www.microsoft.com/download/details.aspx?id=56617) to allow signing all office documents.
Those Subject Interface packages are DLLs that get registered with the operating system and are only available for the x86 platform which means an x64 signtool will not be able to use them.
As a result, with the current situation, either we can use the CNG (x64) or we can use Office SIPS (x86) but we can't use them at the same time.

I thus believe it would be beneficial for most users to have an x86 version of the CNG.