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
We do still need this. We need to be able to determine whether it was us who dynamically created the container (in which case we can remove it when instances are 0), or if it was a passed in element, such as an asp.net form. If the latter, we do not want to remove it from the DOM. 

Make sense?