Add windows_package support

- Add documentation for windows_package resource to README.md
- Add tests for windows_package resource
- Add support for windows_package resource
Squashed commit of the following:

Updated spec/chefspec/matchers/windows_package_spec.rb to RSpec 3 matchers per Seth, prettied up a bit.

updated lib/chefspec/matchers/package.rb per Seth's feedback. Thanks for the tip!
Squashed commit of the following:

Fixed up package.rb

    Updated to RSPEC3 syntax per Seth, cleaned up a bit.

    Per @sethvargo: "Can you refactor this to use the RSpec 3 matchers and new syntax we are moving toward?

    A few things:

    Do not start it blocks with "should". Declare instead
    Don't use should prefix. Use expect() syntax instead
    Follow this kind of pattern for testing a resource matcher"
Use RSpec 3 matchers