REF: Implement RelativeDeltaOffset
RelativeDeltaOffset (basically the same as the existing DateOffset class) is a bit of an odd duck _attributes-wise, this is just a move without yet cdef-ing it.
hmm can you rebase after all of the merges. maybe ABCTimestamp -> _Timestamp as needed. Rebased+green true-green once more rebase i think these were back of queue rebased+green