Fix #359 - Introduce build system based on Gulp

- Add Gulp and new dependencies to `package.json`
- Add `gulpfile.js` with four tasks:
    - `tsc`: For building TypeScript sources
    - `bundle`: For bundling JavaScript modules in a monolith
    - `sorcery`: For resolving the source map chains back to the original TypeScript files
    - `build` (`default`): Runs the whole `tsc` → `bundle` → `sorcery` chain
- Clean up `Dockerfile`, since `cpio` is not needed any more
- Clean up not needed dependencies from `package.json`
- Remove `bin/build`
- Update `bin/release` to use `npm run build` instead of `./bin/build`

src/xterm.js

@@ -10,13 +10,13 @@
  * @license MIT
  */
 
-import { CompositionHelper } from './CompositionHelper.js';
-import { EventEmitter } from './EventEmitter.js';
-import { Viewport } from './Viewport.js';
-import { rightClickHandler, pasteHandler, copyHandler } from './handlers/Clipboard.js';
-import { CircularList } from './utils/CircularList.js';
+import { CompositionHelper } from './CompositionHelper';
+import { EventEmitter } from './EventEmitter';
+import { Viewport } from './Viewport';
+import { rightClickHandler, pasteHandler, copyHandler } from './handlers/Clipboard';
+import { CircularList } from './utils/CircularList';
 import { C0 } from './EscapeSequences';
-import { CharMeasure } from './utils/CharMeasure.js';
+import { CharMeasure } from './utils/CharMeasure';
 import * as Browser from './utils/Browser';
 import * as Keyboard from './utils/Keyboard';