Api and docs

@wesleykendall This commit adds a number of functions to make object-tracking in localized recurrences easier.

Notably, it adds a method `check_due` to the `LocalizedRecurrence` model, which takes a list of objects, and will tell you (with as few runs to the database as possible) which of them are past due. This will be the primary method that we will use to interact with the object tracking. We can pass in any entities that are could potentially fire a trigger, and it will return only those who haven't already fired in the given period.

It also includes a pretty big addition to the documentation, explaining the secondary use case of Localized Recurrence as a method of tracking events, with a brief section on tracking multiple objects.