Backbone's design allows for both static (unchanging, not ES6 static)

and dynamic (function-based) values for many class attributes. To safely

query these, callers are expected to access these attributes using

_.result(obj, 'attrname'), which doesn't feel natural with ES6

classes.
This change introduces accessors for these attributes. You can now fetch

myModel.getURL() or myView.getTagName(), for example, instead of

_.result(myModel, 'url') or _.result(myView, 'tagName').
These are type-safe as well.

Unit tests pass.