fixed and improved JS scaffolding

[swey]

Fixes the initialization of the JS for modules

Refactoring:

• the name, events and default options are defined as constants or at least static (again), in the latest version the variables were recreated for every instance/every call of ClassName.events
• the import statement was moved to the top of the file (more common)
• The class comment is not longer preserved
• the extra line for "export default" was removed and replaced with "export default class"
• ES6 string substitution used
• some JSdoc was added (accessors)
• the destroy method was moved up

[backflip]

@christiansany, can you have a look at this when working on https://github.com/unic/estatico-nou/issues/3?