Support standard MongoDB connection strings

Added support for using standard MongoDB connection strings rather than the existing non-standard "gridfs://" connection string format. The motivation was to provide a way for specifying client options, such as connection pool max size, min size, etc. The options supported depend on the mongo-java-driver in use; the connection string is simply passed through to the driver. The constant for the "gridfs://" URI prefix has been marked as deprecated. I think support for the non-standard connection string format should be removed in a future release, as the standard connection string format supports everything support by the non-standard format. But for now, these changes are intended to be backwards-compatible.