6.0.0

I'm extremely stupid and didn't commit as I go. To anyone reading this
I'm extremely sorry. A lot of these changes are very broad and I plan on
releasing Babel 6.0.0 today live on stage at Ember Camp London so I'm
afraid I couldn't wait. If you're ever in London I'll buy you a beer
(or assorted beverage!) to make up for it, also I'll kiss your feet and
give you a back massage, maybe.

packages/babel-plugin-transform-strict-mode/src/index.js

@@ -0,0 +1,17 @@
+import * as t from "babel-types";
+
+export default function () {
+  return {
+    visitor: {
+      Program(path) {
+        let { node } = path;
+        
+        for (let directive of (node.directives: Array<Object>)) {
+          if (directive.value.value === "use strict") return;
+        }
+
+        path.unshiftContainer("directives", t.directive(t.directiveLiteral("use strict")));
+      }
+    }
+  };
+}