Reverse complement of a sequence and possible supporting reverse complemented coordinates

@ahwagner raised in the VRS/VCF meeting the idea of reverse complementing sequences and making them available in refget. The idea being if you are going to refer to an event on the opposing strand, such as a transcript, then asking for regions in the relative coordinate system would make sense.

From a basic refget POV this would require knowing what the sequence's reverse complement checksum is, converting that into the forward orientation alongside the requested coordinates and then reverse complementing the response.

However this does make the refget `/sequence/checksum` endpoint somewhat more complicated and brings with it additional semantics that a server may or may not support. This could be communicated via service-info. This problem might not be a seqcol issue but recording the reverse complement checksum could be an additional array.