L.Layer and L.Evented docs

Document the new `L.Layer` and `L.Evented` classes for 1.0. This documents some of major changes for in https://github.com/Leaflet/Leaflet/blob/master/CHANGELOG.md#layers-refactoring.

I wasn't quite sure where to put the docs for `L.Layer`, I've added them here under "Utility" but They might be better documented under "ILayer" but I'll leave it split up for now and we can discuss.

Can @mourner and/or @danzel review?