Release of v2.1.0

v2.1 adds support for public methods, a new destroy method, custom callbacks for init, render and destroy, and small bugfixes.

* Reorganized core to allow for new public methods
* Added destroy method to core to allow for breaking down the plugin and it's dom changes
* Added the option to add callback functions to init, render and destroy
* Squashed a bug when wrapping sub-sections in their `div`
* Updated core init in test for easier reuse
* Fixed Grunt `test:browser` task to only run in the browser
* Added destroy tests to core module and a new module for the callbacks