Added clear() and all() to reactive-dict

Added a `clear()` and an `all() method to the `ReactiveDict`.

The second returns a dictionary with all values, and depends on every key (both current and future). Added tests for the two.

A couple of notes:

1. Could add a `.unset(key)` and use it in `.clear`. Haven't needed that yet.

2. Not sure if unsetting a key should invalidate `equals(key, undefined)`?