Modularise JS source in 'lib' subdirectory.

Generate public API documention via jsdoc comments.

lib/index.js

@@ -0,0 +1,17 @@
+'use strict';
+
+const Sharp = require('./constructor');
+[
+  'input',
+  'resize',
+  'composite',
+  'operation',
+  'colour',
+  'channel',
+  'output',
+  'utility'
+].forEach(function (decorator) {
+  require('./' + decorator)(Sharp);
+});
+
+module.exports = Sharp;