hold security descriptors read from OOXML independently

If anyone knows how to interpret that stuff and convert between
BIFF/OOXML feel free to implement..

According to ISO/IEC 29000 the securityDescriptor is application
defined, but recommends that username@domain be used, which is addr-spec
of RFC 822. Well, what Excel2013 writes is something like this
'O:WDG:WDD:(A;;CC;;;S-1-5-21-2328606873-448790680-2149751550-1003)'

Change-Id: I138fa116b019f200c211272abf69831bc49127ab
bit 0 is bit 0

They say they use big-endian bit diagrams, but just to confuse you.

Change-Id: I7fd6bb58c721f1dca2f6ecbaae90d516746762f0
resize vector in advance

Obtaining front() from an empty vector is disallowed, even if size was
reserved. So resize it.

Change-Id: Iae2aafd6b7b725ea36bf9c3fb0a2370c7037777f