Test library with Jest

[francoischalifour]

I wrote some tests a long time ago with AVA but was not feeling confident enough to make them publicly available as they were clunky. I decided to use Jest this time and found it easier to test the DOM.

These tests were added to make sure that the lib is in a strong enough state to upgrade and add more features in the future.

Features tested

• [x] The API is correctly exposed
• [x] All kind of selectors are supported
• [x] Options override the selector if no selector is given
• [x] .images is in sync with the library's state
• [x] .update()
• [x] .show()
• [x] .hide()
• [x] .detach()
• [ ] Events are emitted accordingly
• [ ] The position is correct on zoom
• [x] Integration tests with HTMLElement.click()