Fixed hard coded bson module location

The code currently loads bson from 'mongoose/node_modules/mongodb/node_modules/bson' which assumes that mongoose is installed in mongoose-timestamp's node_modules directory, and that mongodb is installed under mongoose's node_modules directory.  Better to set bson as a dependency and load it directly, since those two things are not always true.

Also, mongoose is really not a dependency at all.  You were just loading it to get access to bson.  You only need it for tests.