[WIP] Changing API from add to set

The current API offers a method `add` to set a value for a key. One would assume that `add` actually does add something, instead it really does set the value.

Therefor I propose to change the API (or at least change the implementation and the docs and leave the old `add` API in pleace to not break current applications).

The current PR is a work in progress, meaning that I only changed the API to offer a `set` for both the localStorage and the cookie. If this idea is accepted, I'll additionally do the following:
* Add a `setAtLocalStorage` method and using the `addToLocalStorage` as a wrapper method (again to not do a backwards compability break)
* Add a `setAtCookies` method (same as above)
* Change the docs to mention the `set` API