Provide explicit Bson.toString() method.

Otherwise, opDispatch() will be invoked when trying to
writeln() a Bson object for debugging purposes, leading to
a confusing exception.

I'm not sure whether goign through Json is the best idea,
it just seemed like a convenient fix for the time being.
Merge pull request #468 from klickverbot/bson-tostring

Provide explicit Bson.toString() method.