* Replace D3 v3 definition with D3 v4 (bundle)
* Replaces the legacy D3 v3 definitions in `d3` with a definition file representing the D3 version 4 standard bundle
* Add `package.json` file with legacy dependency to @types/d3 version >=3.5.36 <4.0.0
* Updated tsconfig.json, package.json, versions
* Updated tsconfig.json to control typings resolution for D3 v3
* Updated package.json to use caret notation of D3 typings
* Updated certain affected definitions header comments with version numbers, where version number was missing buit seemed reasonably ascertainable as latest.
* Added comments to each affected definition file with TODO once upgrade to D3 v4 is considered
* Chore changes as per review.
* Removed TODO "upgrade-to-v4" comments
* Removed added empty lines.
* added the `options` field to dagre's `setEdge`
* added a test as well
* changed some style issues according to linter (var -> let)
* exported `dagre-d3` so it is possible to import it with SystemJS
* Atom's TypeScript package now accepts the import
* changed some style issues according to linter (var -> let)
* changed `let` to `const` whenever it is possible.
* changed `let` to `const` whenever it is possible.
* `const` is possible here as well because the object reference does not change.
