feat(core.set) Add set.
feat(core.set) Additional set methods and tests.
fix(data.collection) Fix get() bug for handling invalid ids correctly.
feat(data.collection) Add tags.
fix(data.collection) get() returns error string when called on non-derived source before computing it.
feat(data.collection) Tag methods: get, set, add, remove, toggle tags.
test(data.collection) Add more tests for get/setTags.
docs(data.collection) More docs.
Merge pull request #109 from racker/data-tags

Data tags