+ Ref https://github.com/docker/notary/issues/144
+ Create ChangeIterator interface
+ Implement ChangeIterator interface for memChangeList
+ Implement ChangeIterator interface for fileChangeList
+ Add iterator test case to changelist_test
+ Add iterator test case to file_changelist_test
+ Change func applyChangelist to use iterator per PR comment
+ Remove redundant defer statement in file_changelist_test.go (PR comment)
+ Change Next error check to simple array bounds check (PR comment)
+ Add negative unit test cases to increase code coverage
Signed-off-by: David Williamson <david.williamson@docker.com>