Support multiple custom ('inline') containers with independent notifications

This is a first shot on supporting multiple custom containers with independent notfications.

So if you have two containers and create noties on them, each of them is able to show notifications independently.

The Notification object gets a method 'getReferenceContainer' which returns the jquery-selector that defines the context in which to search for an existing ul-container. In case the Notification is a 'custom' one, we return the selector for the element the noty was created on, otherwise we return the body element as context.
For the inline-layout (`layouts/inline.js`), this means that the selector has to be class-based instead of id-based, so that multiple containers may exist on a page.

It's possible that I'm still missing some obvious spots and maybe there is a better approach in supporting this feature - just let me know.