[core] Change style API

[oliviertassinari]

Propose the following improvements:

• allow all the class names to be extended with the classes property, so we stop using the xxxClassName pattern. That's pretty close to react-css-themr.

// Before
// How can I do that, should a submit a PR to expose a `roundedClassName`? 

// After
<Paper classes={{ rounded: 'my-rounded-implementation' }} />

• remove the duplication between the theme and the styleManager as we can use
  this.context.styleManager.theme over this.context.theme.
• I have migrated one component, the Paper with that new approach. If we are happy with that, I can convert the whole code base.
• Update the docs
• Automatically generate the classes docs
• Migrate the code base
• ref handling and instance method access

[rosskevin]

very nice!

[oliviertassinari]

@rosskevin Thanks for the feedback, I'm going one step further: auto docs generation 💅 .