Make xfail/xpass useable by other test subroutines

Also mark the failing test_stats() tests as expected failures
(see commentary in #179).
Make test_view() -@ tests portable to OS X

BSD (and POSIX) getopt() requires options to appear before
non-option arguments.
view: Free the header once finished
merge,sort: Output BAM, not SAM. Build indexes

instead of throwing an error.