Making the addon customisable and adding event actions

Hi, This is just a little change to make the module more customisable. It is a breaking change because of the wrapping <div> around the button by default now which I think is a good thing but i'm happy to discuss.

I've also added an action handler to the ZeroClipboard client so that we can expose an ember action that happens after something has successfully been copied to clipboard
This should say `file: your-app/templates/components/custom-clipboard.hbs`