Changes to check for DB instance for review

Submitting changes for review for the following subject: https://groups.google.com/forum/?hl=en#!msg/pgsqlite/CSRAdjkNfbY/Oj6kNGWV654J

This was a request to get the constructor object, which would be awkward given the kind of encapsulation used. As an alternative, provide a member that can be checked very quickly for both existence and a value of truth. To check if an object is really an instance of the plugin db object:

    if (!!db.isdatabase) {
      // transactions on db
    } else {
      // oops not a db
    }