Ratio component – Remove support for iframe/embed/object, improve comments and clarity of visual tests.

(brief description / PR title)

Status: **Ready to review**

Reviewers: @kpeatt @mobify-derrick 
Ticket: #44

## Changes
- Remove support for iframe, object and embed as items to constrain (for now)
- Improve code comments
- Improve descriptions for each test

## How to test-drive this PR
- Review diff
- View ratio test html file and check test descriptions to ensure it’s easy to tell what the result should look like, i.e. can you tell whether a given test case passes or fails.