Sam: implementation of mate_alignment_stop()

* fix off-by-one bug on Sam::alignment_stop (it was reporting the base after the last inclusive base, contradicting the docs)
* refactor cigar operator parsing routine from SamBuilder to Cigar class for mutual use
* add consumes_read_bases and consumes_reference_bases as static methods to the Cigar class
* add tests to the new API and all new functions

This sets the base for issue #6
This call to missing() is a good indication that mate_alignment_stop() needs to handle the case where MC is missing internally -- otherwise we end up calling `string_tag("MC")` twice, once in client code and once in gamgee.