Add create_remote_file_if_missing matcher

The `create_remote_file` matcher doesn't work if the action is `:create_if_missing`. Most of my `remote_file` resources use this action, so it would be useful to have a matcher for this. (And chefspec 0.9.0 didn't check the action attribute so I was able to use that matcher.)

This addresses part of issue #118.

My ruby-fu is not strong; feedback/suggestions welcome.