Adding destroy function and tests

Added destroy function.

Status: **Ready to merge**

Reviewers: @kpeatt @tedtate 
JIRA: https://mobify.atlassian.net/browse/CSOPS-950
Linked PRs: https://github.com/mobify/pinny/pull/61

## Changes
- added `destroy` function
- updated tests with `destroy` function

## How to test-drive this PR
- checkout this branch
- `grunt test`
- `grunt serve` and check out the example page
- in the console, call `destroy` on a pinny and ensure it's destroyed
Is this still relevant now that we have the instance counter?