Checkums are not used properly and force can have strange effects

The logic in `retrieve_artifact!` leaves something to be desired. 

* You can end up with nothign happening when you use the `force` attribute to redeploy the same artifact because of the initial `if` condition.
* `Chef::ChecksumCache` might need to be used better depending on the path we go down. At the moment, the simplest case should be handled with the remote_file resource. Others might have bugs.

Should refactor the logic in this method so that it is more clear. Possibly clear up the logging as well. This may also include some sort of implementation for #28.

Edit: It appears that the class is also now `Chef::Digester` in Chef > 11.0