New way to add toplevel stores, and api merged with global

Updates and breaking changes

• Better way to {addstore} and use mixins and local api. addStore('name-store') now returns {mixin, api} instead of a mixin.

• Merged {api} into {global}, so no more import {api} from 'vuex+';. Use import {global} from 'vuex+'; ` instead.

The demo and tutorial will be updated shortly. Check out the new features here: https://github.com/presidenten/vuex-plus-demo