AllergyIntolerance.onset should be mapped to w5 when.init and appear before attestedDate

Per http://wiki.hl7.org/index.php?title=DSTU_QA_Guidelines

QA 5e: The W5 column must be filled in with all matching elements. w5 elements not present in the resource should be evaluated as to whether they make sense for the resource and fall within the 80%

QA 6b: Data elements & search criteria within the same "family" should be ordered in the same way (particularly within the same "family"). Data elements should in general be ordered in a "sensible" way - related elements together, more important elements towards the top, "big" elements (those that take a lot of space in instances) towards the bottom. W5 elements must appear in the order defined unless there are strong business reasons for a different order (raise with MnM to get permission to override the sort rule for a given resource). Note that items related to W5 items can still be inter-mixed with W5 items