Integrate attributes with ES6

[ArnaudBuchholz]

Part of #222, #303

As a developer, I want to use attributes in an ES6 class declaration because I don't want to use gpf.define

Acceptance criterias

• It should be able to create a class (using Function or class) and then pass it to a method that will read and use attributes
• It should work with inheritance

[ArnaudBuchholz]

One idea could be to simulate ES6 decorators by having a 'sort of' transpiler plugged into:

• gpf.require.define (while loading sources)
• NodeJS' require (see https://github.com/elastic/require-in-the-middle)
• browsers (need to analyze how blanket.js does its magic)

[ArnaudBuchholz]

Actually, only gpf.require.define will be adapted since it is compatible cross platform.

[ArnaudBuchholz]

Used babel-eslint on specific folders only (since it breaks the no-unused-vars rule)

[ArnaudBuchholz]

Had to deeply refactor the way entityDefinition are built and provided some for standard objects

[ArnaudBuchholz]

Need to improve class definition importing:

• Create a sub class of entity definition that does not validate name

[ArnaudBuchholz]

_GpfImportedClassDefinition