Cleanin a bit some asserts + adding more test cases including some failing ones
Fix make lint
Reformating
Fix unit tests (and issue with empty prefixes)

See http://stackoverflow.com/questions/11865043/mongoengine-query-operator-for-negation-substitute-for-exclude/19554990#19554990 :)
Remove Link.created_at
Filter on short_path instead (some might not have a prefix...) and also remove created_at
OPS-1416 Adding warning for unindexed query
Merge pull request #17 from olivierlefloch/olefloch/OPS-1416/stop_storing_short_path

Olefloch/ops 1416/stop storing short path