Use html5 template tag instead of script/div hack

[charlie-hadden]

I ran into this issue on a project I'm currently working on when using ckeditor in a nested field.

This PR gets rid of the script/div hack in favour of HTML5's <template> tag. This works well for us as we don't need to target IE, but I'm not sure how suitable you'd find it for merging upstream. The browser support isn't too great: http://caniuse.com/#feat=template

That said, there are numerous polyfills that could be used alongside the changes here. The only thing I'd be concerned about with those is whether they block execution of any <script> tags inside the <template>. I've not had chance to test this out yet. Perhaps it would make sense to add an option to nested_fields_for to opt-in to <template>?

Also had a quick look through the open issues, and I believe it would also fix #63.

[ncri]

Thanks for this! I will give it a try and test it a bit when I have some time. I won't merge it with master yet because of the lack of support in IE, but we could maintain a separate branch. Or maybe better, make this optional. If you'd make using a template tag optional and use the old hackish way by default I could merge this straight away. We could have an option like use_template_tag for the nested_fields_for call and add a note about it in the readme.