Make 'er quack like an Icalendar Component!

[jon-sully]

In the Icalendar gem there is a Component class that many other classes inherit from, including Calendar and Event. The Component specifies a :parent attribute that is available on all of its inheritors. This is useful throughout the usage of the Icalendar gem since you can pass around a single object and still have access to most of the object tree if you need it. Occurrence does not follow that pattern, requiring that any app passing around occurrences (either as an individual or an object) must also accompany that occurrence with the associated event if it wants any context. I don't think it's necessary to go so far as to make Occurrence inherit from Component, as I want to be wary of too much overhead on an otherwise very simple class / struct, but I think Occurrence could at least implement the :parent method since that information is readily available at the time the Occurrences are instantiated in Schedule.

This PR adds and populates the :parent method on the Occurrence object with a very simplistic change and adds a an example to the specs to make sure that the field is populated.

[jon-sully]

@jnraine this is the follow up from the one I closed briefly. Should be squared up now 👍

[rahearn]

Thank you!