Add ability to define settings externally

By defining an `observatory-settings` package which defines a global `ObservatorySettings`, you can now configure should_publish, [allow](docs.meteor.com/#allow), and control the logger constructor settings.

Ex.

```javascript
ObservatorySettings = {
    allow: {
        insert: function (userId, doc) {
            //make sure the userId is correct
            return !(doc.uid && doc.uid !== userId);
        },
        update: function (userId, doc, fields, modifier) {
            return false;
        },
        remove: function (userId, doc) {
            return false;
        }
    },
    log_http: false,
    printToConsole: false,
    should_publish: function (publish) {
        //only publish logs to the admins
        return Meteor.users.findOne({ _id: publish.userId, role: "admin"});
    }
};
```

This should resolve https://github.com/jhoxray/observatory/issues/21.