Responsiveness is currently based on the use of an iframe, however this method causes performance issues and could be troublesome when used with ad blockers. So make sure that the user is still able to create a chart without iframe when responsive is false.
For convenience, charts are now automatically released after each spec if they are not acquired using `persistent: true`. Also remove the confusing and error prone `chartInstance` global variable and make sure that chart instances are local to each spec.
Since we changed the way how meta data are stores, now unit tests need to work on real Chart instances. This commit brings some helpers to inject/cleanup HTML canvas and it's wrapper into/from the DOM.