Implementing GUID using boost

I haven't taken a look at the hash table stuff, so I didn't touch that, but I was able to get rid of our (respectable!) guid random number seeding stuff in `guid_init`, etc. I ran the test suite and uncovered one problem which I was able to fix. I also started the program with a moderately sized chart and transactions (my mom's business's books), and everything seems good.
That doesn't work because `gen` isn't defined there. Looks like

    GncGUID guid;
    guid_new (&guid);
    return guid;
fits the bill?