Bunch of fixes which should really be separate commits/branches:

* Added an option for customizing the close button text/symbol (see #10)
* added a CSS classname ("cookiebanner-close") to the element containing the close button (which should further help customizations)
* fixed 'data-' options handling which wasn't really working as it should've been (on browsers implementing dataset)
* switched to using camelCase option names within the code (due to the dataset API converting automatically and it being easier to replicate for browsers that don't support dataset)
* avoid "polluting" the global namespace and instantiating Cookiebanner if we're not being invoked in a way that really requires it
* added 2 simple demos within the tests folder for easier manual testing