CXX-265 Configure logging through startup options

A few comments/questions:

-For cleanup of appenders, there's a clearAppenders() function, and this will actually delete the appenders.  Though it seems clear that when you attach the appender you hand ownership of it over to the log domain, users using these options won't see that call, and I wonder if they'd be surprised by this behavior.  Also, since we're not using the appender handles at all, or allowing the user to use them, I wasn't sure how important it was to call clearAppenders() at all.

-Where should configureLoggingOptions() live?  Options.cpp seems like an ok option, but I think there's probably a better fit.  If I add a cleanup function that should probably live in the same place.

-Should there be a way to configure, through options, specific minimum logged severity levels for different logging domains?  If you were doing it manually, you'd be able to set it by component.
We require that users synchronize their own appenders. This test injects an appender that could, potentially, be used by background threads (or at least I think so). So, I see two potential issues:

- If a background thread happened to log something during the window where your tests run, the count might change unexpectedly.
- The _count variable isn't synchronized.

I think the latter is solvable by using our Atomic types. The former is harder; not sure what you should do about that, exactly.