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
If there are no cigar operators that consume reference bases, won't we end up returning mate_alignment_start() - 1 (same as for as missing tag)? Is this what we want?