v1.3.0: Close image, examples and wrapper added

- added attribute data-tooltip-close-img to insert an image in close button
- added a wrapper for close button text ```<span class="$tooltip_prefix_class + 'tooltip__closetext__container"``` in order to make it stylable 
- added another example "It’s customisable" on https://a11y.nicolas-hoffmann.net/dialog-tooltip/
- another feature: when you click on a button that launches a modal tooltip, the class <code>is-active</code> is added to this button.
Closed #5

1st release of script

1st release of this jQuery accessible dialog tooltip window https://a11y.nicolas-hoffmann.net/dialog-tooltip/