Fixes #22 - Implemented Guid class

Implemented ::Guid class, which is compatible with Win32/COM GUID
(Proven by unit-tests in test/unit-guid.cc, all of which are passing
at the moment).

Potential issues: endian-ness of Guid::__data4 -- The standard
dictates that this should always be stored in big-endian order,
however I believe I am not doing this correctly, and currently do not
have a non-little endian system to test this against. Even so, I don't
expect the Guid class to be used except on Windows systems, which
should always be little-endian unless they're cellphones or something.
Merge pull request #22 from caitp/issue-22

[enhance] GUID class, with win32/COM compatibility.