Add update event method

This all function can be done by:

```
eventSchema.statics.update = function(type, date, properties) {
    return Event.findOneQ({
        'type': type,
        'date': date
    })
    .then(function(e) {
        if (!e) throw "Event doens't exists";

        e.properties = properties;
        e.markModified("properties");

        return e.saveQ().thenResolve(e);
    });
};
```