refs #5936 started to work on removing files from core and the plugins directories that are no longer present in the new build
refs #5936 added many tests for the new filesystem methods to make sure they only delete the files they are supposed to do
refs #5936 added some phpdoc blocks
refs #5936 moved some business logic from controller into model which also brings the advantage to make it better testable. Controller should be always simple/small... removeGoneFiles() is not really testable as it uses the static filesystem and I did not want to make the code ugly to mock it... Looking forward to dependency injection
refs #5936 forgot to commit the tests
Merge pull request #5984 from piwik/5936_removeGoneFilesDuringUpdate

refs #5936 remove gone files during update