3.0 - Paginator templates part 1

This is the first part of the [proposed changes](https://github.com/markstory/cakephp/wiki/Paginatorhelper-cleanup) for PaginatorHelper and ultimately the other core helpers. I've only update `next()` and `prev()` so far as I wanted to get feedback, make sure I was on the right path before doing the other methods. I've made the default templates use `<li>` elements, this makes it really easy to use with both bootstrap and foundation. Other methods would need to start using li elements as well for everything to work. Some specific things I'd like feedback on:

* I've switched from `{foo}` to `{{foo}}` for placeholders. I thought reusing mustache/twig output braces. This syntax means there would be one fewer placeholder styles to learn/use. Another option is to use `{:foo}` that `String::insert()` already uses. Alternatively, we could make `String` consistent with everything else.
* Almost all of the HTML attribute generation has been removed, this should work well for PaginatorHelper. In other helpers we may still need to have generated attributes that go into a `attributes` placeholder.
* I'm not totally sold on the class name of `StringTemplate`, any suggestions would be great.
* Are there any options that were removed that would still be useful?
* I've intentionally left template options out of next/prev as I generally make all my pagination elements the same, which is what having one set of templates would make easy.

After this is merged, I'm going to start working on the other methods in PaginatorHelper.