Commit generated default dependencies as they're read

Closes #1284. 

- Move `this._computedBindings = {}` out of `_setupComputes` and into
  `setup`
- Overwrite `_get` in `_setupDefaults` to set defaults with `this.attr`
  when they're read in defaultGenerators
- Prevent the calling of a defaultGenerator if its value is passed in on
  instantiation
- Create a test for reading defaults with `this.attr` in
  defaultGenerators
- Create a test for defining defaults on instantiation
Why is this needed?