sam header parsing is not spec compliant

[Dan Kortschak]

During development of hts/sam I made a horrendous and unfortunate error
in reading the SAM spec, resulting in SAM/BAM header parsing being
non-spec compliant.

https://github.com/biogo/hts/issues/26

I've discussed this with one other user/dev and think that the last
option in the list at the issue is the way to fix this, but I would like
to get this right. So any input into the approach that people feel would
be best to take is welcomed.

Dan