Adding isModern test and property on browser object

Adding isModern test and property on browser object, in addition to an `is-modern-browser`

Status: **Ready to merge**

Reviewers: @tedtate @kpeatt 

## Changes
- added `browser.isModern` property
- added `is-modern-browser` class on body
- added tests

## How to test-drive this PR
- checkout branch
- `grunt test`
- `grunt serve` and test that this works in various browsers