Hotfix: Missing CommonJS convenience files for core build artifacts

Described in #1636 - this includes the addition of lib/core/ CommonJS convenience files, which make it easier to require or import Fine Uploader core builds. The ES6 & CommonJS feature docs page includes updated information that explains how to easily import core files.

I also found some typos in the examples on that doc page that illustrate how to import Fine Uploader directly using a <script> tag. In many cases, one directory entry was missing. That too is now fixed.

Build improvements

A complete re-write of the build code was the center of this release, completing cases #1569, #1581, #1605, and #1607.

High-level improvements & changes:

• Grunt and all grunt-related dependencies are gone, replaced with a single Makefile.
• Updated & enhanced build instructions in the README.
• "Core" build artifacts distributed with each endpoint type. Core JS output files exclude all UI-centric modules.
• A standalone drag-and-drop module exists in the build output. This is included inside of each endpoint directory in the downloadable zip files, and is available inside of a specific directory in the artifacts published to npm. Using CommonJS? require('fine-uploader/lib/dnd'). Using ES6 imports? import Dnd from 'fine-uploader/lib/dnd'.
• All JS and CSS build artifacts (minified and non-minified) include source map files.
• Any Node.js version may now be used to build Fine Uploader (no longer tied to ancient versions of Node).
• Downloadable zip files are now located in GitHub releases (instead of AWS S3).