Sam: implementation of mate operations

* 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

fixes #6
you're right, I'll add a special treatment to it like I did to unclipped stop to only deduct 1 when something got added to it.