Add windows_package support

Note: This might be a little controversial because the windows_package resources technically lives in an LWRP in the Windows cookbook. However, the windows_* resources are steadily merging into Chef mainline, and even as LWRP's these resources see common enough use that I think they should be included in Chefspec. 

- Add documentation for windows_package resource to README.md
- Add tests for windows_package resource
- Add support for windows_package resource