initial refactor of retrieve_from_nexus
copy only_if it doesnt exist or isnt the same file
add docs to the new method
Merge branch 'test_kitchen' into caching_refactors
add a new attributes for deleting on force
make conditionals clearer
Merge pull request #64 from RiotGames/caching_refactors

Checkums are not used properly and force can have strange effects