Layer refactoring

[mourner]

A work in progress. This PR contains major refactoring of Leaflet layers architecture, creating a base Layer class for all layers to inherit from, and putting all shared layers logic there (as well as layers-related logic in Map.js), DRYing up and simplifying the code significantly, removing ugly hacks, as well as addressing #496 #1249 #1742 and other issues. A part of the major refactoring for Leaflet 1.0 #2210.

• Added Layer class which all layers added to a map should inherit from.
• Added add and remove events to all layers.
• Added pane option to all layers that can be changed (e.g. you can set pane: 'overlayPane' to a tile layer).
• Added shadowPane option to markers as well.
• Added getEvents method to all layers that returns an {event: listener, ...} hash; layers now manage its listeners automatically without having to do this in onAdd/onRemove.
• Improved performance of adding/removing layers with layers control present (instead of listening to any layer add/remove, the control only listens to layers added in configuration).
• Fixed FeatureGroup getBounds to work correctly when containing circle markers.
• Removed Map tilelayersload event.
• Removed Popup open and close events in favor of add and remove for consistency.
• Moved all layer-related logic in Map.js to Layer.js.

[mourner]

@danzel @tmcw @jfirebaugh now merged in master, take a look, or just skim through the summary of changes in the description.

[danzel]

Some brackets here would make this a bit more readable:
var paneName = name ? (this.options[name] || name) : this.options.pane;

[mourner]

Yep, you're right :)

[danzel]

Read through, looks good

[mourner]

Thanks!