Allow more flexible DOM structure

I tried including iDangerous Swiper in a project that uses [Backbone.js](http://backbonejs.org) with [Marionette.js](http://marionettejs.com).
Since Backbone is very wrapper-happy and cannot be convinced to not wrap views into their own <div>s (cf. [https://github.com/documentcloud/backbone/issues/546#issuecomment-3604746](https://github.com/documentcloud/backbone/issues/546#issuecomment-3604746) and [http://grokbase.com/t/gg/backbone-marionette/134ps3q9s2/extra-wrappers-how-to-avoid](http://grokbase.com/t/gg/backbone-marionette/134ps3q9s2/extra-wrappers-how-to-avoid)), iDangerous's expectation for a fixed DOM structure - with no itermediate <div>s between container, wrapper and slides - makes it unusable for my project. I have at least two intermediary <div>s between .swiper-wrapper and .swiper-slide and several more between .swiper-container and .swiper-wrapper.

To alleviate this, I've simply changed the way iDangerous finds the wrapper and the slides. Have a look below.