Classes split (flowjs#315)

* Split up Flow.js into distinct classes and rely upon build-time assembly.

- `$` shorthand symbol was removed in favor of `this` and context has been taken care of (including for event handlers)
- _real_ class methods replace the former simili-constructor syntax
- started replacing a couple `each()` by `for..of` when it helped getting rid of significant context switch complexity.
- FlowFile and evalOpts (counter-intuitive) bindings into Flow namespace had to be dirtily reproduced because the Grunt/Karma/Jasmine hardly support full ES6 import within spec files.
- tests adapted
- flow.support is removed
  1. Because if there is no `window`, user are on their own and there is no reason keeping node.js from loading the code
  2. Because `support` rather depend on the transpilation and polyfills part of the final build issues by the toolchain
  => Avoid cluttering bootstrap with a runtime check.
* fix onDrop.bind() which was then impossible to remove

rollup.config.js

@@ -18,7 +18,7 @@ let plugins = [
 
 export default [
     {
-        input: 'src/flow.js',
+        input: 'src/Flow.js',
         plugins,
         output: {
             name: 'Flow',
@@ -28,7 +28,7 @@ export default [
 	},
     },
     {
-        input: 'src/flow.js',
+        input: 'src/Flow.js',
         plugins: plugins.concat([terser()]),
         output: {
             name: 'Flow',