Add options to connect to databases rather than only localhost:27017

[faridnsh]

In my project, we don't use environmental variables and usually our databases aren't on the same servers. So it'll be nice if we could pass a Server or ReplSet instance or maybe a string that is just a URL.

[faridnsh]

It'll also be nice to just pass an existing DB object, so if we use the native driver beside GridFS, we just share the same connection.