Check the exact startup mode of a windows service

Usage;

````ruby
describe service("Winmgmt") do
    it{ should have_start_mode("Manual") }
end
````

Additionally I've implemented "be_installed" on the Service resource - just to check that the service has been registered, regardless of its startmode or current run state.

````ruby
describe service("Winmgmt") do
    it{ should be_installed }
end
````