Add full support for es6

Converts the tool chain to use babel, eslint, and webpack vs. the previous proprietary solutions.

Additionally begins enforcing additional linting concerns as well as updates the code to reflect these rules.

Fixes #855
Fixes #993

lib/handlebars.runtime.js

@@ -1,15 +1,15 @@
-/*globals Handlebars: true */
-module base from "./handlebars/base";
+/*global window */
+import * as base from './handlebars/base';
 
 // Each of these augment the Handlebars object. No need to setup here.
 // (This is done to easily share code between commonjs and browse envs)
-import SafeString from "./handlebars/safe-string";
-import Exception from "./handlebars/exception";
-module Utils from "./handlebars/utils";
-module runtime from "./handlebars/runtime";
+import SafeString from './handlebars/safe-string';
+import Exception from './handlebars/exception';
+import * as Utils from './handlebars/utils';
+import * as runtime from './handlebars/runtime';
 
 // For compatibility and usage outside of module systems, make the Handlebars object a namespace
-var create = function() {
+function create() {
   var hb = new base.HandlebarsEnvironment();
 
   Utils.extend(hb, base);
@@ -24,7 +24,7 @@ var create = function() {
   };
 
   return hb;
-};
+}
 
 var Handlebars = create();
 Handlebars.create = create;